Documentation/sphinx: build the media intermediate rst files for all outputs

This is a stopgap measure to allow building outputs other than html.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Markus Heiser <markus.heiser@darmarit.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>