Remove tox-linters job, the default job now runs on bionic, so there's
no need to use an extra job. Replace it with the default job.
Replace obsolete build-openstack-sphinx-docs with new template
build-openstack-docs-pti which calls "tox -e docs" directly.
Update tox.ini for this to use python3.
Fix URL in docs to use new location of repo after OpenDev rename.
Change-Id: I8c46d95ae1156af01eb07fe7559bbf9e751cf055
Assures that the root README.me file renders images correctly on both
GitHub and Sphinx builded docs.
This should also allow us to review doc changes on zuul by loaded
their output.
Change-Id: I6cd54be73bfe25a75296af4984821320ebad7d97