Added notifications documentation page

In this changeset, I added a new Sphinx directive (from Nova) that
allows us to automatically display notification samples for of
all the upcoming notifications in Watcher.

Partially Implements: blueprint watcher-notifications-ovo

Change-Id: Id7d819ee21db6dc616c78faea483b883f77e3bd6

doc/source/dev/notifications.rst

@@ -0,0 +1,13 @@
+..
+      Except where otherwise noted, this document is licensed under Creative
+      Commons Attribution 3.0 License.  You can view the license at:
+
+          https://creativecommons.org/licenses/by/3.0/
+
+.. _watcher_notifications:
+
+========================
+Notifications in Watcher
+========================
+
+.. versioned_notifications::