docs: networking: convert vrf.txt to ReST

- add SPDX header;
- adjust title markup;
- Add a subtitle for the first section;
- mark code blocks and literals as such;
- adjust identation, whitespaces and blank lines;
- add to networking/index.rst.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/index.rst

@@ -113,6 +113,7 @@ Contents:
    tproxy
    tuntap
    udplite
+   vrf
 
 .. only::  subproject and html
 

MAINTAINERS

@@ -18106,7 +18106,7 @@ M:	David Ahern <dsahern@kernel.org>
 M:	Shrijeet Mukherjee <shrijeet@gmail.com>
 L:	netdev@vger.kernel.org
 S:	Maintained
-F:	Documentation/networking/vrf.txt
+F:	Documentation/networking/vrf.rst
 F:	drivers/net/vrf.c
 
 VSPRINTF