Publish docs on release

Also set the in-development section title for release notes so
they don't appear under a bogus version number.

Change-Id: Ie882a8b13bbb844660198fee2cb1e0ba46b95343

.zuul.yaml

@@ -149,3 +149,7 @@
     experimental:
       jobs:
         - nodepool-functional-py35-debian-src
+    release:
+      jobs:
+        - publish-zuul-docs
+        - publish-openstack-sphinx-docs-infra-python3

doc/source/releasenotes.rst

@@ -3,3 +3,4 @@
 ===============
 
 .. release-notes::
+   :unreleased-version-title: In Development