Documentation/Changes: add minimal requirements for documentation build

As discussed at linux-doc ML, the best is to keep all documents
backward compatible with Sphinx version 1.2, as it is the latest
version found on some distros like Debian.

All books currently support it.

Please notice that, while it mentions the eventual need of
XeLaTex and texlive to build pdf files, this is not a minimal
requirement, as one could just be interested on building html
documents. Also, identifying the minimal requirements for
texlive packages is not trivial, as each distribution seems to
use different criteria on grouping LaTex functionalities.

While here, update the current kernel version to 4.x.

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