media: Documentation: add documentation of Intel IPU6 driver and hardware overview

Add a documentation for an overview of IPU6 hardware and describe the main
the components of IPU6 driver.

Signed-off-by: Bingbu Cao <bingbu.cao@intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>