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>
diff --git a/net/ipv6/Kconfig b/net/ipv6/Kconfig
index 2ccaee9..5a6111d 100644
--- a/net/ipv6/Kconfig
+++ b/net/ipv6/Kconfig
@@ -13,7 +13,7 @@
 	  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