doc-rst: add stand-alone conf.py to gpu folder

With the gpu/conf.py, the gpu folder can be build and distributed
stand-alone. To compile only the html of 'gpu' folder use::

  make SPHINXDIRS="gpu" htmldocs

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