media: docs: v4l2-subdev: move "Subdev registration" to a subsection

The subdev registration topic is pretty lengthy, and takes more than
half of the "V4L2 sub-devices" section. Help readers in finding their
way through the document by dedicating a subsection to
"Subdev registration".

Each of the two registration methods takes many paragraphs, but since
adding a subsubsection would be overkill, just emphasize them in bold.

Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>