[media] docs-rst: media: Push CEC documentation under CEC section

The CEC documentation added two sections on the main media kAPI level.
There should be only one.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>