scripts/documentation-file-ref-check: detect broken :doc:`foo`

As we keep migrating documents to ReST, we're starting to see
more of such tags.

Right now, all such tags are pointing to a documentation file,
but regressions may be introduced.

So, add a check for such kind of issues as well.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>