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/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