docs: networking: device drivers: convert chelsio/cxgb.txt to ReST

docs: networking: device drivers: convert chelsio/cxgb.txt to ReST

- add SPDX header;
- use copyright symbol;
- adjust titles and chapters, adding proper markups;
- comment out text-only TOC from html/pdf output;
- mark code blocks and literals as such;
- add notes markups;
- mark tables as such;
- mark lists as such;
- adjust identation, whitespaces and blank lines where needed;
- add to networking/index.rst.

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