The sync from https://review.opendev.org/733244 updated to
openstackdocstheme 2.2.1 and reno 3.1.0 versions.
Set openstackdocs_pdf_link to link to PDF file. Note that
the link to the published document only works on docs.openstack.org
where the PDF file is placed in the top-level html directory. The
site-preview places the PDF in a pdf directory.
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.
See also
http://lists.openstack.org/pipermail/openstack-discuss/2020-May/014971.html
Change-Id: I63feca3be7a797b2c2af8e650b938aef0f1cd782
bedf5758b1