extended-controls.rst: split up per control class

The extended-controls.rst file had become too big. Split it up: each
control class reference gets its own rst file, and this file just
describes the Extended Control API.

Each control class reference is also moved up one level into the
table of contents to make it easier to find e.g. the codec control
reference.

Finally I rearranged the order so that all camera-related control
classes are grouped together, ditto for codec/jpeg and fm-rx/tx.

The ext-ctrls-codec.rst is still pretty big and it is a candidate
to split up further in the future, possibly per codec.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>