staging: remove isdn capi drivers

As described in drivers/staging/isdn/TODO, the drivers are all
assumed to be unmaintained and unused now, with gigaset being the
last one to stop being maintained after Paul Bolle lost access
to an ISDN network.

The CAPI subsystem remains for now, as it is still required by
bluetooth/cmtp.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20191210210455.3475361-1-arnd@arndb.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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>