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.
Depends-On: https://review.opendev.org/728938
Change-Id: Ia31f69f00eb24771bf700b4e25ae73a09c1c8aed
0fbce17220