docs: bridge: add STP doc

Add STP part for bridge document.

Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>