drm/bridge: Add documentation sections

The bridge documentation overview is quite packed already, and we'll add
some more documentation that isn't part of an overview at all.

Let's add some sections to the documentation to separate each bits.

Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20210910101218.1632297-2-maxime@cerno.tech