docs: pdf: add all Documentation/*/index.rst to PDF output

Currently, all index files should be manually added to the
latex_documents array at conf.py.

While this allows fine-tuning some LaTeX specific things, like
the name of the output file and the name of the document, it
is not uncommon to forget adding new documents there.

So, add a logic that will seek for all Documentation/*/index.rst.
If the index is not yet at latex_documents, it includes using
a reasonable default.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>