media: cec-core.rst: include cec-pin.h and cec-notifier.h

Include the CEC pin framework documentation by reading cec-pin.h.
Include the CEC notifier framework documentation by reading cec-notifier.h.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>