docs: networking: convert generic_netlink.txt to ReST

Not much to be done here:
- add SPDX header;
- add a document title;
- add to networking/index.rst.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/generic_netlink.txt → Documentation/networking/generic_netlink.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+===============
+Generic Netlink
+===============
+
 A wiki document on how to use Generic Netlink can be found here:
 
  * http://www.linuxfoundation.org/collaborate/workgroups/networking/generic_netlink_howto

Documentation/networking/index.rst

@@ -60,6 +60,7 @@ Contents:
    fore200e
    framerelay
    generic-hdlc
+   generic_netlink
 
 .. only::  subproject and html