media: admin-guide: Add an introduction chapter

The media's admin guide is currently just a group of
not-connected docs.

Add an introduction chapter for it to start making sense
to a random reader.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/admin-guide/media/index.rst

@@ -34,6 +34,8 @@ Video4Linux (V4L)  driver-specific documentation
 	:maxdepth: 5
 	:numbered:
 
+	intro
+
 	cardlist
 
 	v4l-with-ir

Documentation/admin-guide/media/intro.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+============
+Introduction
+============
+
+The media subsystem consists on Linux support for several different types
+of devices:
+
+- Audio and video grabbers;
+- PC and Laptop Cameras;
+- Complex cameras found on Embedded hardware;
+- Analog and digital TV;
+- HDMI Customer Electronics Control (CEC);
+- Multi-touch input devices;
+- Remote Controllers;
+- Media encoders and decoders.
+
+Due to the diversity of devices, the subsystem provides several different
+APIs:
+
+- Remote Controller API;
+- HDMI CEC API;
+- Video4Linux API;
+- Media controller API;
+- Video4Linux Request API (experimental);
+- Digital TV API (also known as DVB API).