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. A
couple of variables are also not needed anymore, remove them.
Set openstackdocs_pdf_link to link to PDF file.
Remove docs requirements from lower-constraints, they are not installed.
For more info. please refer [1].
[1] https://review.opendev.org/728938
Change-Id: I90d064e63baa5f067f38a881eb9a556dfb435e97
f8658d9880