sphinx-pre-install: add support for Mageia

Add support for detecting and installing missing packages
on Mageia. I opted to use "urpmi" at the install instructions,
as this is present on Mageia since ever. Yet, if I were using
Mageia 6, I would likely be using "dnf", as it is, IMHO,
easier to use.

Tested with Mageia 6.

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