Switch to "modern" way of building docs using sphinx-build directly,
remove now unsed parts from setup.cfg.
Upgrade to openstackdocstheme 1.20 and remove obsolete variables from
conf.py.
Convert external links to internal RST links so that Sphinx can verify
that they are correct.
Replace redirected links with new targets.
Use opendev.org instead of github.com where appropriate.
Change-Id: Iedcc008b170821aa74acefc02ec6a243a0dc307c
8dd89e52d0