Documentation: add cleanmediadocs to the documentation targets

This was broken when updating the documentation targets for the Sphinx
build, and moving from %docs target pattern to explicitly listed
targets.

Cc: Markus Heiser <markus.heiser@darmarit.de>
Cc: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Fixes: 22cba31b

 ("Documentation/sphinx: add basic working Sphinx configuration and build")
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>