This adds support for pdf documentation generation. Work items:
* Provide a distinct index for pdf doc. This is required as the toctree in a
tab fails to render as a pdf document.
* Remove the v2 API reference from the pdf documentation. "rest_expand_all",
"rest_method" and "rest_parameters" are not supported.
* Add a pdf-docs tox env
* Update conf.py
Story: 2006075
Task: 34808
Change-Id: I848d59b551fe6d5077960dc45554c7281c2372a7
b7e8323f63