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>

Documentation/networking/device_drivers/index.rst

@@ -31,6 +31,7 @@ Contents:
    3com/vortex
    amazon/ena
    aquantia/atlantic
+   chelsio/cxgb
 
 .. only::  subproject and html
 

drivers/net/ethernet/chelsio/Kconfig

@@ -26,7 +26,7 @@ config CHELSIO_T1
 	  This driver supports Chelsio gigabit and 10-gigabit
 	  Ethernet cards. More information about adapter features and
 	  performance tuning is in
-	  <file:Documentation/networking/device_drivers/chelsio/cxgb.txt>.
+	  <file:Documentation/networking/device_drivers/chelsio/cxgb.rst>.
 
 	  For general information about Chelsio and our products, visit
 	  our website at <http://www.chelsio.com>.