Update .zuul.yaml to use PTI job, update tox.ini to use the common way
to run sphinx, update setup.cfg for handling warnings as errors.
Add new docs target to tox.ini and make it default target, the target is
needed by CI scripts.
Note that ossa does not work with python3, so the docs environment does
not use basepython. This needs a rework of doc/source/_exts/vmt.py.
Prepare for repository publication by:
- moving all YAML under ossa/
- cargo-culting doc building from openstack/governance
- adding a LICENSE (CC 3.0 BY)