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.
Add configuration for openstackdocstheme 'report a bug'.
Depends-On: https://review.opendev.org/728938
Change-Id: I0938ef80ec161607e7c4b2a4a846f1b32e7614e3
74943a2aa1