media: MAINTAINERS: Split sensors and lens drivers, add documentation

Split lens drivers from camera sensor into a new section. This makes it
easier to maintain the list.

Also add documentation related files under the camera sensor entry. Add
the word "CAMERA" to the subject as well since there are many other kinds
of sensors.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>