Start publishing docs with using the proper template.

For this, add also a doc/requirements.txt file and move docs
requirements there. Update tox.ini for this and use the modern way to
invoke sphinx-build for building.
Remove obsolete sectoins from setup.cfg.
Update the conf.py files and remove unused lines
with newer openstackdocstheme - and require newer theme.

Fix indents in CONTRIBUTING.rst so that the formatting works nicely.

Also, use lower-constraints template instead of individual jobs.

Change-Id: Ia1a7030878f12ca0b8184363173eada929ece3bd