build-openstack-sphinx-docs jobs are obsolete since some time,
use the newer build-openstack-docs-pti jobs instead using a template.
Also, update tox.ini to use sphinx-build following PTI.
Switch to openstackdocstheme, since oslosphinx is dead.
Use doc/requirements.txt for doc jobs.
Fix doc building errors.
Remove now obsolete sections from setup.cfg