doc-rst: generic way to build PDF of sub-folders

This extends the method to build only sub-folders to the targets
"latexdocs" and "pdfdocs". To do so, a conf.py in the sub-folder is
required, where the latex_documents of the sub-folder are
defined. E.g. to build only gpu's PDF add the following to the
Documentation/gpu/conf.py::

  +latex_documents = [
  +    ("index", "gpu.tex", "Linux GPU Driver Developer's Guide",
  +     "The kernel development community", "manual"),
  +]

and run:

  make SPHINXDIRS=gpu pdfdocs

Signed-off-by: Markus Heiser <markus.heiser@darmarIT.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>