docs: admin-guide: add auxdisplay files to it after conversion to ReST

Those two files describe userspace-faced information. While part of
it might fit on uAPI, it sounds to me that the admin guide is the
best place for them.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>