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.
This commit add PDF building tox.ini environment and options for LaTeX
output. overview.rst is copied from README.rst. And, the original
README.rst file is shrunk because we don't need such a long information
in the README file. People can see the same contents in overview.rst