switch documentation job to new PTI

This is a mechanically generated patch to switch the documentation
jobs to use the new PTI versions of the jobs as part of the
python3-first goal.

See the python3-first goal document for details:
https://governance.openstack.org/tc/goals/stein/python3-first.html

Change-Id: I31736abc085feb76ca292bfdda73109461c11105
Story: #2002586
Task: #24312

.zuul.yaml

@@ -3,17 +3,17 @@
       - openstack-python-jobs
       - openstack-python35-jobs
       - check-requirements
-      - release-notes-jobs
+      - release-notes-jobs-python3
     check:
       jobs:
         - openstack-tox-lower-constraints
         - openstack-tox-cover
-        - build-openstack-sphinx-docs
+        - openstack-tox-docs
     gate:
       jobs:
         - openstack-tox-lower-constraints
         - openstack-tox-cover
-        - build-openstack-sphinx-docs
+        - openstack-tox-docs
     post:
       jobs:
         - publish-openstack-sphinx-docs