doc-rst: add media/conf_nitpick.py

doc-rst: add media/conf_nitpick.py

The media/conf_nitpick.py is a *build-theme* wich uses the nit-picking
mode of sphinx. To compile only the html of 'media' with the nit-picking
build use::

  make SPHINXDIRS=media SPHINX_CONF=conf_nitpick.py htmldocs

With this, the Documentation/conf.py is read first and updated with the
configuration values from the Documentation/media/conf_nitpick.py.

The origin media/conf_nitpick.py comes from Mauro's experimental
docs-next branch::

  https://git.linuxtv.org/mchehab/experimental.git mchehab/docs-next

BTW fixed python indentation in media/conf_nitpick.py.  Python
indentation is 4 spaces [1] and Python 3 disallows mixing the use of
tabs and spaces for indentation [2].

[1] https://www.python.org/dev/peps/pep-0008/#indentation
[2] https://www.python.org/dev/peps/pep-0008/#tabs-or-spaces

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