docs: isdn: convert to ReST and add to kAPI bookset

The ISDN documentation is a mix of admin guide, uAPI and kAPI.

Ideally, it should be split. Yet, not sure if it would worth
the troble. Anyway, we have the same kind of mix on several
drivers specific documentation. So, just like the others, keep
the directory at the root Documentation/ tree, just adding a
pointer to it at the kAPI section, as the documentation was
written with the Kernel developers in mind.

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