docs-rst: enable the Sphinx math extension

This extension will be used by the media books.

The name of the math image extension changed on Sphinx 1.4.x,
according with:
	http://www.sphinx-doc.org/en/stable/ext/math.html#module-sphinx.ext.imgmath



Let's autodetect, to keep building with versions < 1.4.

Suggested-by: Markus Heiser <markus.heiser@darmarit.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>