This patch enables building .pdf documentation using new 'pdf-docs'
tox environment. The newly created tox environment creates latex
source codes using sphinx-build and then builds the pdf
documentation using make.
Story: 2006070
Task: 35467
Change-Id: Ie2b3df9f09445ca4f71bb1fbf69c92a4d7b3e719
f46106c2f5