Switch to openstackdocstheme 2.2.0 and reno 3.1.0 versions. Using
these versions will allow especially:
* Linking from HTML to PDF document
* parallelizing building of documents
Update Sphinx version as well.
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_pdf_link to link to PDF file.
Set openstackdocs_auto_name to use 'project' as name.
Depends-On: https://review.opendev.org/728938
Change-Id: I76a21521eeb432c3f4089048a4635719ba6d785c
a2efd0b82a