Switch to newer openstackdocstheme and reno versions

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

Update Sphinx version as well.

Create doc/requirements.txt file and use it, 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: Ic3503a8b38e67e50669428b36f6581cb980d687f

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