[BEAM-2103] Documenting Python 3 support

website/src/_includes/section-menu/documentation.html

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       </li>
     </ul>
   </li>   
-  </ul>      
+
+  <li class="section-nav-item--collapsible">
+    <span class="section-nav-list-title">Java</span>
+
+    <ul class="section-nav-list">
+      <li><a href="{{ site.baseurl }}/documentation/transforms/java/overview/">Overview</a></li>
+      <li class="section-nav-item--collapsible">
+        <span class="section-nav-list-title">Element-wise</span>
+
+        <ul class="section-nav-list">
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/elementwise/filter/">Filter</a></li>
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/elementwise/flatmapelements/">FlatMapElements</a></li>  
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/elementwise/keys/">Keys</a></li>
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/elementwise/kvswap/">KvSwap</a></li> 
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/elementwise/mapelements/">MapElements</a></li>
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/elementwise/pardo/">ParDo</a></li>   
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/elementwise/partition/">Partition</a></li>
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/elementwise/regex/">Regex</a></li>
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/elementwise/reify/">Reify</a></li>
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/elementwise/tostring/">ToString</a></li>
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/elementwise/withkeys/">WithKeys</a></li> 
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/elementwise/withtimestamps/">WithTimestamps</a></li>  
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/elementwise/values/">Values</a></li>                                            
+        </ul>
+      </li>
+      <li class="section-nav-item--collapsible">
+        <span class="section-nav-list-title">Aggregation</span>
+
+        <ul class="section-nav-list">
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/aggregation/approximatequantiles/">ApproximateQuantiles</a></li>
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/aggregation/approximateunique/">ApproximateUnique</a></li>
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/aggregation/cogroupbykey/">CoGroupByKey</a></li>
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/aggregation/combine/">Combine</a></li>
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/aggregation/combinewithcontext/">CombineWithContext</a></li>
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/aggregation/count/">Count</a></li>
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/aggregation/distinct/">Distinct</a></li>
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/aggregation/groupbykey/">GroupByKey</a></li>
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/aggregation/groupintobatches/">GroupIntoBatches</a></li>
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/aggregation/latest/">Latest</a></li>
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/aggregation/max/">Max</a></li>
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/aggregation/mean/">Mean</a></li>
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/aggregation/min/">Min</a></li>
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/aggregation/sample/">Sample</a></li>
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/aggregation/sum/">Sum</a></li>
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/aggregation/top/">Top</a></li>
+        </ul>
+      </li>
+      <li class="section-nav-item--collapsible">
+        <span class="section-nav-list-title">Other</span>
+
+        <ul class="section-nav-list">
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/other/create/">Create</a></li>
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/other/flatten/">Flatten</a></li>
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/other/passert/">PAssert</a></li>
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/other/view/">View</a></li>
+          <li><a href="{{ site.baseurl }}/documentation/transforms/java/other/window/">Window</a></li>
+        </ul>
+      </li>
+    </ul>
+  </li>   
+  </ul>
+
 </li>
 
 <li class="section-nav-item--collapsible">

website/src/documentation/transforms/java/aggregation/approximatequantiles.md

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+---
+layout: section
+title: "ApproximateQuantiles"
+permalink: /documentation/transforms/java/aggregation/approximatequantiles/
+section_menu: section-menu/documentation.html
+---
+<!--
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+-->
+# ApproximateQuantiles
+<table align="left">
+    <a target="_blank" class="button"
+        href="https://beam.apache.org/releases/javadoc/current/index.html?org/apache/beam/sdk/transforms/ApproximateQuantiles.html">
+      <img src="https://beam.apache.org/images/logos/sdks/java.png" width="20px" height="20px"
+           alt="Javadoc" />
+     Javadoc
+    </a>
+</table>
+<br>
+Takes a comparison function and the desired number of quantiles *n*, either
+globally or per-key. Using an approximation algorithm, it returns the
+minimum value, *n-2* intermediate values, and the maximum value.
+
+## Examples
+**Example**: to compute the quartiles of a `PCollection` of integers, we
+would use `ApproximateQuantiles.globally(5)`. This will produce a list
+containing 5 values: the minimum value, Quartile 1 value, Quartile 2
+value, Quartile 3 value, and the maximum value.
+
+## Related transforms 
+* [ApproximateUnique]({{ site.baseurl }}/documentation/transforms/java/aggregation/approximateunique)
+  estimates the number of distinct elements or distinct values in key-value pairs
+* [Combine]({{ site.baseurl }}/documentation/transforms/java/aggregation/combine)

website/src/documentation/transforms/java/aggregation/approximateunique.md

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+---
+layout: section
+title: "ApproximateUnique"
+permalink: /documentation/transforms/java/aggregation/approximateunique/
+section_menu: section-menu/documentation.html
+---
+<!--
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+-->
+# ApproximateUnique
+<table align="left">
+    <a target="_blank" class="button"
+        href="https://beam.apache.org/releases/javadoc/current/index.html?org/apache/beam/sdk/transforms/ApproximateUnique.html">
+      <img src="https://beam.apache.org/images/logos/sdks/java.png" width="20px" height="20px"
+           alt="Javadoc" />
+     Javadoc
+    </a>
+</table>
+<br>
+Transforms for estimating the number of distinct elements in a collection
+or the number of distinct values associated with each key in a collection
+of key-value pairs.
+
+## Examples
+See [BEAM-7703](https://issues.apache.org/jira/browse/BEAM-7703) for updates.
+
+## Related transforms 
+* [Count]({{ site.baseurl }}/documentation/transforms/java/aggregation/count)
+  counts the number of elements within each aggregation.
+* [Distinct]({{ site.baseurl }}/documentation/transforms/java/aggregation/distinct)

website/src/documentation/transforms/java/aggregation/cogroupbykey.md

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+---
+layout: section
+title: "CoGroupByKey"
+permalink: /documentation/transforms/java/aggregation/cogroupbykey/
+section_menu: section-menu/documentation.html
+---
+<!--
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+-->
+# CoGroupByKey
+<table align="left">
+    <a target="_blank" class="button"
+        href="https://beam.apache.org/releases/javadoc/current/index.html?org/apache/beam/sdk/transforms/join/CoGroupByKey.html">
+      <img src="https://beam.apache.org/images/logos/sdks/java.png" width="20px" height="20px"
+           alt="Javadoc" />
+     Javadoc
+    </a>
+</table>
+<br>
+Aggregates all input elements by their key and allows downstream processing
+to consume all values associated with the key. While `GroupByKey` performs
+this operation over a single input collection and thus a single type of
+input values, `CoGroupByKey` operates over multiple input collections. As
+a result, the result for each key is a tuple of the values associated with
+that key in each input collection.
+
+See more information in the [Beam Programming Guide]({{ site.baseurl }}/documentation/programming-guide/#cogroupbykey).
+
+## Examples
+**Example**: Say you have two different files with user data; one file has
+names and email addresses and the other file has names and phone numbers.
+
+You can join those two data sets, using the username as a common key and the
+other data as the associated values. After the join, you have one data set
+that contains all of the information (email addresses and phone numbers)
+associated with each name.
+
+```java
+PCollection<KV<UID, Integer>> pt1 = /* ... */;
+PCollection<KV<UID, String>> pt2 = /* ... */;
+
+final TupleTag<Integer> t1 = new TupleTag<>();
+final TupleTag<String> t2 = new TupleTag<>();
+PCollection<KV<UID, CoGBKResult>> result =
+  KeyedPCollectionTuple.of(t1, pt1).and(t2, pt2)
+    .apply(CoGroupByKey.create());
+result.apply(ParDo.of(new DoFn<KV<K, CoGbkResult>, /* some result */>() {
+  @ProcessElement
+  public void processElement(ProcessContext c) {
+    KV<K, CoGbkResult> e = c.element();
+    CoGbkResult result = e.getValue();
+    // Retrieve all integers associated with this key from pt1
+    Iterable<Integer> allIntegers = result.getAll(t1);
+    // Retrieve the string associated with this key from pt2.
+    // Note: This will fail if multiple values had the same key in pt2.
+    String string = e.getOnly(t2);
+    ...
+}));
+```
+
+## Related transforms 
+* [CombineWithContext]({{ site.baseurl }}/documentation/transforms/java/aggregation/combinewithcontext) to combine elements.
+* [GroupByKey]({{ site.baseurl }}/documentation/transforms/java/aggregation/groupbykey) takes one input collection.

website/src/documentation/transforms/java/aggregation/combine.md

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+---
+layout: section
+title: "Combine"
+permalink: /documentation/transforms/java/aggregation/combine/
+section_menu: section-menu/documentation.html
+---
+<!--
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+-->
+# Combine
+<table align="left">
+    <a target="_blank" class="button"
+        href="https://beam.apache.org/releases/javadoc/current/index.html?org/apache/beam/sdk/transforms/Combine.html">
+      <img src="https://beam.apache.org/images/logos/sdks/java.png" width="20px" height="20px"
+           alt="Javadoc" />
+     Javadoc
+    </a>
+</table>
+<br>
+A user-defined `CombineFn` may be applied to combine all elements in a
+`PCollection` (global combine) or to combine all elements associated
+with each key. 
+
+While the result is similar to applying a `GroupByKey` followed by
+counting the number of values in each `Iterable`, there is an impact
+on the code you must write as well as the performance of the pipeline.
+Writing a `ParDo` that counts the number of elements in each value
+would be very straightforward. However, as described in the execution
+model, it would also require all values associated with each key to be
+processed by a single worker. This introduces a lot of communication overhead.
+Using a `CombineFn` requires the code be structured as an associative and
+commumative operation. But, it allows the use of partial sums to be precomputed.
+
+See more information in the [Beam Programming Guide]({{ site.baseurl }}/documentation/programming-guide/#combine).
+
+## Examples
+**Example 1**: Global combine
+Use the global combine to combine all of the elements in a given `PCollection`
+into a single value, represented in your pipeline as a new `PCollection` containing
+one element. The following example code shows how to apply the Beam-provided
+sum combine function to produce a single sum value for a `PCollection` of integers.
+
+```java
+// Sum.SumIntegerFn() combines the elements in the input PCollection. The resulting PCollection, called sum,
+// contains one value: the sum of all the elements in the input PCollection.
+PCollection<Integer> pc = ...;
+PCollection<Integer> sum = pc.apply(
+   Combine.globally(new Sum.SumIntegerFn()));
+```
+
+**Example 2**: Keyed combine
+Use a keyed combine to to combine all of the values associated with each key
+into a single output value for each key. As with the global combine, the
+function passed to a keyed combine must be associative and commutative.
+
+```java
+// PCollection is grouped by key and the Double values associated with each key are combined into a Double.
+PCollection<KV<String, Double>> salesRecords = ...;
+PCollection<KV<String, Double>> totalSalesPerPerson =
+  salesRecords.apply(Combine.<String, Double, Double>perKey(
+    new Sum.SumDoubleFn()));
+// The combined value is of a different type than the original collection of values per key. PCollection has
+// keys of type String and values of type Integer, and the combined value is a Double.
+PCollection<KV<String, Integer>> playerAccuracy = ...;
+PCollection<KV<String, Double>> avgAccuracyPerPlayer =
+  playerAccuracy.apply(Combine.<String, Integer, Double>perKey(
+    new MeanInts())));
+```
+
+```java
+# PCollection is grouped by key and the numeric values associated with each key
+# are averaged into a float.
+player_accuracies = ...
+{% github_sample /apache/beam/blob/master/sdks/python/apache_beam/examples/snippets/snippets_test.py tag:combine_per_key
+%}
+```
+
+## Related transforms 
+* CombineWithContext
+* GroupByKey

website/src/documentation/transforms/java/aggregation/combinewithcontext.md

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+---
+layout: section
+title: "CombineWithContext"
+permalink: /documentation/transforms/java/aggregation/combinewithcontext/
+section_menu: section-menu/documentation.html
+---
+<!--
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+-->
+# CombineWithContext
+<table align="left">
+    <a target="_blank" class="button"
+        href="https://beam.apache.org/releases/javadoc/curent/index.html?org/apache/beam/sdk/transforms/CombineWithContext.html">
+      <img src="https://beam.apache.org/images/logos/sdks/java.png" width="20px" height="20px"
+           alt="Javadoc" />
+     Javadoc
+    </a>
+</table>
+<br>
+A class of transforms that contains combine functions that have access to `PipelineOptions` and side inputs through `CombineWithContext.Context`.
+
+## Examples
+See [BEAM-7703](https://issues.apache.org/jira/browse/BEAM-7703) for updates.
+
+## Related transforms 
+* [Combine]({{ site.baseurl }}/documentation/transforms/java/aggregation/combine)
+  for combining all values associated with a key to a single result