CI: save site+docs builds as artifacts

Makes the downloadable for previewing locally.

.github/workflows/ci.yml

@@ -118,6 +118,19 @@ jobs:
         source activate pandas-dev
         doc/make.py --warnings-are-errors | tee sphinx.log ; exit ${PIPESTATUS[0]}
 
+    - name: Save website as an artifact
+      uses: actions/upload-artifact@v2
+      with:
+        name: website
+        path: web/build
+        retention-days: 14
+    - name: Save docs as an artifact
+      uses: actions/upload-artifact@v2
+      with:
+        name: docs
+        path: doc/build/html
+        retention-days: 14
+
     # This can be removed when the ipython directive fails when there are errors,
     # including the `tee sphinx.log` in te previous step (https://github.com/ipython/ipython/issues/11547)
     - name: Check ipython directive errors

doc/source/development/contributing.rst

@@ -629,6 +629,17 @@ the documentation are also built by Travis-CI. These docs are then hosted `here
 <https://pandas.pydata.org/docs/dev/>`__, see also
 the :ref:`Continuous Integration <contributing.ci>` section.
 
+Previewing changes
+------------------
+
+Once, the pull request is submitted, GitHub Actions will automatically build the
+documentation. To view the built site:
+
+#. Wait for the ``CI / Web and docs`` check to complete.
+#. Click ``Details`` next to it.
+#. From the ``Artifacts`` drop-down, click ``docs`` or ``website`` to download
+   the site as a ZIP file.
+
 .. _contributing.code:
 
 Contributing to the code base