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.