Switch to openstackdocstheme 2.2.0 and reno 3.1.0 versions. Using
these versions will allow especially:
* Linking from HTML to PDF document
* Allow parallel building of documents
* Fix some rendering problems
Update Sphinx version as well.
Remove docs requirements from lower-constraints, they are not needed
during install or test but only for docs building.
openstackdocstheme renames some variables, so follow the renames
before the next release removes them. A couple of variables are also
not needed anymore, remove them.
Set openstackdocs_auto_name to use 'project' as name.
Depends-On: https://review.opendev.org/728938
Change-Id: I31eada6a7e2628c16441adf54f3c2b97b9375103
877043c538