docs: networking: convert cxacru.txt to ReST

- add SPDX header;
- add a document title;
- mark code blocks and literals as such;
- mark lists as such;
- adjust identation, whitespaces and blank lines;
- add to networking/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>