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>