Add step to release checklist for verifying documentation builds.

The webhooks aren't always reliable so it's good to manually check
that the docs have been rebuilt before publishing an announcement.

release_checklist.txt

@@ -50,7 +50,12 @@ Release Checklist
       Iterate version number to differentiate development version
       from latest release.
 
-13. Publish update announcement to relevant mailing lists:
+13. Make sure the documentation reflects the new versions:
+
+    * https://datatest.readthedocs.io/ (stable)
+    * https://datatest.readthedocs.io/en/latest/ (latest)
+
+14. Publish update announcement to relevant mailing lists:
 
   python-announce-list@python.org
   testing-in-python@lists.idyll.org