Publish documentation at every release

We normally publish documentation updates in the promote pipeline,
using documentation built in the gate. As a result, the published
docs tend to lag behind release events until the next post-release
change lands. Run a separate docs build and publish it on release,
so that release notes and version information included in the
documentation reflect the most recent tag prior to the next change
merging (which for stable projects like this one, could be a while).
Override the tag jobvar to force publication to the branch path
rather than using a version-specific tree.

Change-Id: Ibf1ecfa0420a3c170beb79c14a9a9ef13fea9046
Depends-On: https://review.opendev.org/838525

.zuul.yaml

@@ -21,3 +21,4 @@
     release:
       jobs:
         - opendev-release-python
+        - opendev-publish-unversioned-tox-docs