media: docs: move uAPI book to userspace-api/media

Since 2017, there is an space reserved for userspace API,
created by changeset 1d596dee38 ("docs: Create a user-space API guide").

As the media subsystem was one of the first subsystems to use
Sphinx, until this patch, we were keeping things on a separate
place.

Let's just use the new location, as having all uAPI altogether
will likely make things easier for developers.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/userspace-api/media/dvb/ca.rst

@@ -0,0 +1,32 @@
+.. Permission is granted to copy, distribute and/or modify this
+.. document under the terms of the GNU Free Documentation License,
+.. Version 1.1 or any later version published by the Free Software
+.. Foundation, with no Invariant Sections, no Front-Cover Texts
+.. and no Back-Cover Texts. A copy of the license is included at
+.. Documentation/userspace-api/media/fdl-appendix.rst.
+..
+.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
+
+.. _dvb_ca:
+
+####################
+Digital TV CA Device
+####################
+
+The Digital TV CA device controls the conditional access hardware. It
+can be accessed through ``/dev/dvb/adapter?/ca?``. Data types and and ioctl
+definitions can be accessed by including ``linux/dvb/ca.h`` in your
+application.
+
+.. note::
+
+   There are three ioctls at this API that aren't documented:
+   :ref:`CA_GET_MSG`, :ref:`CA_SEND_MSG` and :ref:`CA_SET_DESCR`.
+   Documentation for them are welcome.
+
+.. toctree::
+    :maxdepth: 1
+
+    ca_data_types
+    ca_function_calls
+    ca_high_level