Add links to other repos and match dropdowns to docs site (#653)

- renamed **Documentation** drop-down menu to **Docs**
- added **Source** drop-down menu with links to source for different
implementations
- added Swift to the **Source** and **Docs** menus

---------

Co-authored-by: Ian Cook <ianmcook@gmail.com>

Author: thisisnic

_includes/header.html

@@ -21,14 +21,13 @@
           <div class="dropdown-menu" aria-labelledby="navbarDropdownGetArrow">
             <a class="dropdown-item" href="{{ site.baseurl }}/install/">Install</a>
             <a class="dropdown-item" href="{{ site.baseurl }}/release/">Releases</a>
-            <a class="dropdown-item" href="https://github.com/apache/arrow">Source Code</a>
           </div>
         </li>
         <li class="nav-item dropdown">
           <a class="nav-link dropdown-toggle" href="#"
              id="navbarDropdownDocumentation" role="button" data-toggle="dropdown"
              aria-haspopup="true" aria-expanded="false">
-             Documentation
+             Docs
           </a>
           <div class="dropdown-menu" aria-labelledby="navbarDropdownDocumentation">
             <a class="dropdown-item" href="{{ site.baseurl }}/docs">Project Docs</a>
@@ -46,6 +45,31 @@
             <a class="dropdown-item" href="{{ site.baseurl }}/docs/r">R</a>
             <a class="dropdown-item" href="https://github.com/apache/arrow/blob/main/ruby/README.md">Ruby</a>
             <a class="dropdown-item" href="https://docs.rs/arrow/latest">Rust</a>
+            <a class="dropdown-item" href="https://github.com/apache/arrow/blob/main/swift/Arrow/README.md">Swift</a>
+          </div>
+        </li>
+        <li class="nav-item dropdown">
+          <a class="nav-link dropdown-toggle" href="#"
+             id="navbarDropdownSource" role="button" data-toggle="dropdown"
+             aria-haspopup="true" aria-expanded="false">
+             Source
+          </a>
+          <div class="dropdown-menu" aria-labelledby="navbarDropdownSource">
+            <a class="dropdown-item" href="https://github.com/apache/arrow">Main Repo</a>
+            <hr/>
+            <a class="dropdown-item" href="https://github.com/apache/arrow/tree/main/c_glib">C GLib</a>
+            <a class="dropdown-item" href="https://github.com/apache/arrow/tree/main/cpp">C++</a>
+            <a class="dropdown-item" href="https://github.com/apache/arrow/tree/main/csharp">C#</a>
+            <a class="dropdown-item" href="https://github.com/apache/arrow-go">Go</a>
+            <a class="dropdown-item" href="https://github.com/apache/arrow-java">Java</a>
+            <a class="dropdown-item" href="https://github.com/apache/arrow-js">JavaScript</a>
+            <a class="dropdown-item" href="https://github.com/apache/arrow-julia">Julia</a>
+            <a class="dropdown-item" href="https://github.com/apache/arrow/tree/main/matlab">MATLAB</a>
+            <a class="dropdown-item" href="https://github.com/apache/arrow/tree/main/python">Python</a>
+            <a class="dropdown-item" href="https://github.com/apache/arrow/tree/main/r">R</a>
+            <a class="dropdown-item" href="https://github.com/apache/arrow/tree/main/ruby">Ruby</a>
+            <a class="dropdown-item" href="https://github.com/apache/arrow-rs">Rust</a>
+            <a class="dropdown-item" href="https://github.com/apache/arrow/tree/main/swift">Swift</a>
           </div>
         </li>
         <li class="nav-item dropdown">

index.html

@@ -14,7 +14,7 @@ <h3>Format</h3>
   <div class="col-lg-4 mt-3">
       <h3>Libraries</h3>
       <p>Arrow's libraries implement the format and provide building blocks for a range of <a href="{{ site.baseurl }}/use_cases/">use cases</a>, including high performance analytics. <a href="{{ site.baseurl }}/powered_by/">Many popular projects</a> use Arrow to ship columnar data efficiently or as the basis for analytic engines.</p>
-      <p>Libraries are available for <a href="{{ site.baseurl }}/docs/c_glib/">C</a>, <a href="{{ site.baseurl }}/docs/cpp/">C++</a>, <a href="https://github.com/apache/arrow/blob/main/csharp/README.md">C#</a>, <a href="https://godoc.org/github.com/apache/arrow/go/arrow">Go</a>, <a href="{{ site.baseurl }}/docs/java/">Java</a>, <a href="{{ site.baseurl }}/docs/js/">JavaScript</a>, <a href="{{ site.baseurl }}/julia/">Julia</a>, <a href="https://github.com/apache/arrow/blob/main/matlab/README.md">MATLAB</a>, <a href="{{ site.baseurl }}/docs/python/">Python</a>, <a href="{{ site.baseurl }}/docs/r/">R</a>, <a href="https://github.com/apache/arrow/blob/main/ruby/README.md">Ruby</a>, and <a href="https://docs.rs/crate/arrow/">Rust</a>.
+      <p>Libraries are available for <a href="{{ site.baseurl }}/docs/c_glib/">C</a>, <a href="{{ site.baseurl }}/docs/cpp/">C++</a>, <a href="https://github.com/apache/arrow/blob/main/csharp/README.md">C#</a>, <a href="https://godoc.org/github.com/apache/arrow/go/arrow">Go</a>, <a href="{{ site.baseurl }}/docs/java/">Java</a>, <a href="{{ site.baseurl }}/docs/js/">JavaScript</a>, <a href="{{ site.baseurl }}/julia/">Julia</a>, <a href="https://github.com/apache/arrow/blob/main/matlab/README.md">MATLAB</a>, <a href="{{ site.baseurl }}/docs/python/">Python</a>, <a href="{{ site.baseurl }}/docs/r/">R</a>, <a href="https://github.com/apache/arrow/blob/main/ruby/README.md">Ruby</a>, <a href="https://docs.rs/crate/arrow/">Rust</a>, and <a href="https://github.com/apache/arrow/blob/main/swift/Arrow/README.md">Swift</a>.
       See <a href="{{ site.baseurl }}/install/">how to install</a> and get started.
   </div>
   <div class="col-lg-4 mt-3">