docs: networking: device drivers: convert 3com/3c509.txt to ReST

- add SPDX header;
- adjust titles and chapters, adding proper markups;
- mark code blocks and literals as such;
- add notes markups;
- mark tables 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

@@ -27,6 +27,7 @@ Contents:
    netronome/nfp
    pensando/ionic
    stmicro/stmmac
+   3com/3c509
 
 .. only::  subproject and html