scripts: add a script to check if Documentation/00-INDEX is sane

It is easy to forget adding/removing entries at the
Documentation/00-INDEX file. In a matter of fact, even before
ReST conversion, people use to forget adding things here, as
there are lots of missing stuff out there.

Now that we're doing a hard work converting entries to ReST,
and while this hole file is not outdated, it is good to have
some tool that would help to verify that this file is kept
updated.

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