docs: networking: convert ipv6.txt to ReST

Not much to be done here:

- add SPDX header;
- add a document title;
- mark a literal as such, in order to avoid a warning;
- 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/index.rst

@@ -70,6 +70,7 @@ Contents:
    iphase
    ipsec
    ip-sysctl
+   ipv6
 
 .. only::  subproject and html
 

Documentation/networking/ipv6.rst

@@ -1,9 +1,15 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+====
+IPv6
+====
+
 
 Options for the ipv6 module are supplied as parameters at load time.
 
 Module options may be given as command line arguments to the insmod
 or modprobe command, but are usually specified in either
-/etc/modules.d/*.conf configuration files, or in a distro-specific
+``/etc/modules.d/*.conf`` configuration files, or in a distro-specific
 configuration file.
 
 The available ipv6 module parameters are listed below.  If a parameter