docs: bridge: update doc format to rst

docs: bridge: update doc format to rst

The current bridge kernel doc is too old. It only pointed to the
linuxfoundation wiki page which lacks of the new features.

Here let's start the new bridge document and put all the bridge info
so new developers and users could catch up the last bridge status soon.

In this patch, Convert the doc to rst format. Add bridge brief introduction,
FAQ and contact info.

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>