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 <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/admin-guide/kernel-parameters.txt

···
 			      shot down by NMI
 
 	autoconf=	[IPV6]
-			See Documentation/networking/ipv6.txt.
+			See Documentation/networking/ipv6.rst.
 
 	show_lapic=	[APIC,X86] Advanced Programmable Interrupt Controller
 			Limit apic dumping. The parameter defines the maximal
···
 			miss to occur.
 
 	disable=	[IPV6]
-			See Documentation/networking/ipv6.txt.
+			See Documentation/networking/ipv6.rst.
 
 	hardened_usercopy=
                         [KNL] Under CONFIG_HARDENED_USERCOPY, whether
···
 			to workaround buggy firmware.
 
 	disable_ipv6=	[IPV6]
-			See Documentation/networking/ipv6.txt.
+			See Documentation/networking/ipv6.rst.
 
 	disable_mtrr_cleanup [X86]
 			The kernel tries to adjust MTRR layout from continuous

Documentation/networking/index.rst

···
    iphase
    ipsec
    ip-sysctl
+   ipv6
 
 .. only::  subproject and html
 

Documentation/networking/ipv6.txt Documentation/networking/ipv6.rst

···
+.. 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

net/ipv6/Kconfig

···
 	  For general information about IPv6, see
 	  <https://en.wikipedia.org/wiki/IPv6>.
 	  For specific information about IPv6 under Linux, see
-	  Documentation/networking/ipv6.txt and read the HOWTO at
+	  Documentation/networking/ipv6.rst and read the HOWTO at
 	  <http://www.tldp.org/HOWTO/Linux+IPv6-HOWTO/>
 
 	  To compile this protocol support as a module, choose M here: the