media: Documentation: Split camera sensor documentation

Split camera sensor documentation into user and kernel portions. This
should make it easier for the user space developers to find the relevant
documentation.

Also add a list of exemplary drivers and add imx219 driver to it, besides
those that were already mentioned.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>