media: admin-guide/media/cec.rst: update CEC debugging doc

The documentation on how to create your own Raspberry Pi CEC debugger was a
bit out of date. Update it to the Raspberry Pi 4B, drop the mention of the RTC
and a link to a picture that no longer works.

Also reorganize the text to make it easier to follow and change the pins to
match the pins I use.

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