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
The motivation for this conversion is to have DevStack's docs be
generated using a more familair workflow for OpenStack projects, using
Sphinx.
Changing from raw HTML to RST will also make it easier to contribute
more documentation, as well as making edits less of a hassle.
The majority of the work was done by using Pandoc to convert from HTML
to RST, with minor edits to the output to remove errors in Sphinx.
Change-Id: I9636017965aeade37b950ddf5bdb0c22ab9004bd
