media: docs: add cros-references to the other media guides

In order to make easier for people to navigate between the
three media guides, add cross-references between them
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/admin-guide/media/index.rst

@@ -2,9 +2,21 @@
 
 .. include:: <isonum.txt>
 
+============================
+Media subsystem usage guides
+============================
 
+This section contains usage information about media subsystem and
+its supported drivers.
 
+Please see:
 
+- :doc:`/userspace-api/media/index`
+     for the userspace APIs used on media devices.
+
+- :doc:`/driver-api/media/index`
+     for driver development information and Kernel APIs used by
+     media devices;
 
 .. _uapi-v4l-drivers:
 

Documentation/driver-api/media/index.rst

@@ -6,8 +6,16 @@
 Media subsystem kernel internal API
 ===================================
 
+This section contains usage information about media subsystem and
+its supported drivers.
 
+Please see:
 
+- :doc:`/admin-guide/media/index`
+    for usage information about media subsystem and supported drivers;
+
+- :doc:`/userspace-api/media/index`
+     for the userspace APIs used on media devices.
 
 
 .. only:: html

Documentation/userspace-api/media/index.rst

@@ -6,6 +6,17 @@
 Linux Media Infrastructure userspace API
 ########################################
 
+This section contains the  driver development information and Kernel APIs
+used by media devices.
+
+Please see:
+
+- :doc:`/admin-guide/media/index`
+    for usage information about media subsystem and supported drivers;
+
+- :doc:`/driver-api/media/index`
+     for driver development information and Kernel APIs used by
+     media devices;
 
 
 .. only:: html