This patch adds a `pdf-docs` tox target that will build PDF versions of our docs. As per the Train community goal: https://governance.openstack.org/tc/goals/selected/train/pdf-doc-generation.html Add sphinxcontrib-svg2pdfconverter to doc/requirements.txt to convert our SVGs. Update docs requirement as well to follow PTI. Change-Id: I0b866b2e57557290533ee7e7e162e56c9c17cf54 Story: 2006072