docs: networking: Convert bridge.txt to rst

The kernel documentation is now restructured text. Convert the Ethernet
Bridge documentation and include it in the toplevel kernel
documentation.

 - Fix heading adornments.
 - Add license identifier.

Signed-off-by: Tobin C. Harding <me@tobin.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>