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

Also, add docs tox environment so that the job works.

Change-Id: I8dd911d46623c0185d238b293e0218cfa435ed46
Story: #2002586
Task: #24342

.zuul.yaml

@@ -2,10 +2,10 @@
     templates:
       - openstack-python-jobs
       - openstack-python35-jobs
-      - publish-openstack-sphinx-docs
+      - publish-openstack-docs-pti
       - periodic-stable-jobs
       - check-requirements
-      - release-notes-jobs
+      - release-notes-jobs-python3
     check:
       jobs:
         - trove-functional-mysql

tox.ini

@@ -112,6 +112,9 @@ deps = -r{toxinidir}/requirements.txt
 commands = {[testenv]commands}
            python tools/trove-pylint.py {posargs:check}
 
+[testenv:docs]
+basepython = python3
+commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:lower-constraints]
 basepython = python3