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

Documentation/networking/bridge.txt → Documentation/networking/bridge.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+=================
+Ethernet Bridging
+=================
+
 In order to use the Ethernet bridging functionality, you'll need the
 userspace tools.
 

Documentation/networking/index.rst

@@ -18,6 +18,7 @@ Contents:
    failover
    net_failover
    alias
+   bridge
 
 .. only::  subproject