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/rc/rc-intro.rst

@@ -0,0 +1,31 @@
+.. 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
+
+.. _Remote_controllers_Intro:
+
+************
+Introduction
+************
+
+Currently, most analog and digital devices have a Infrared input for
+remote controllers. Each manufacturer has their own type of control. It
+is not rare for the same manufacturer to ship different types of
+controls, depending on the device.
+
+A Remote Controller interface is mapped as a normal evdev/input
+interface, just like a keyboard or a mouse. So, it uses all ioctls
+already defined for any other input devices.
+
+However, remove controllers are more flexible than a normal input
+device, as the IR receiver (and/or transmitter) can be used in
+conjunction with a wide variety of different IR remotes.
+
+In order to allow flexibility, the Remote Controller subsystem allows
+controlling the RC-specific attributes via
+:ref:`the sysfs class nodes <remote_controllers_sysfs_nodes>`.