Switch to openstackdocstheme 2.2.1 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.
Change pygments_style to 'native' since old theme version always used
'native' and the theme now respects the setting and using 'sphinx' can
lead to some strange rendering.
Depends-On: https://review.opendev.org/729744
Change-Id: I311a5daa382dfca07e618eb6cbb3f44bd0502b02
30199f4f94