docs: networking: convert 6lowpan.txt to ReST

- add SPDX header;
- use document title markup;
- 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>
Reviewed-by: Stefan Schmidt <stefan@datenfreihafen.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>