docs-rst: allow generating some LaTeX pages in landscape

Portrait is too small for some tables used at the media docs.
So, allow documents to tell Sphinx to generate some pages
in landscape by using:

.. raw:: latex

    \begin{landscape}

<some stuff>

.. raw:: latex

    \end{landscape}

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>