media: docs: create vimc documentation

Create vimc documentation file to explain its basic features, its
topology, how to configure it and to document vimc's subdevices.

Signed-off-by: André Almeida <andrealmeid@collabora.com>
Suggested-by: Helen Koike <helen.koike@collabora.com>
[hverkuil-cisco@xs4all.nl: Fix typo: The ``v4l2-utils`` -> The ``v4l-utils`` package]
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>