media: docs: Move the H264 stateless codec uAPI

Now that we've destaged the H264 stateless codec controls,
and with all the pieces in place, update the documentation
and move it to its own section.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>