docs: networking: convert pktgen.txt to ReST

- add SPDX header;
- adjust title markup;
- use bold markups on a few places;
- mark code blocks and literals 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>

samples/pktgen/README.rst

@@ -3,7 +3,7 @@ Sample and benchmark scripts for pktgen (packet generator)
 This directory contains some pktgen sample and benchmark scripts, that
 can easily be copied and adjusted for your own use-case.
 
-General doc is located in kernel: Documentation/networking/pktgen.txt
+General doc is located in kernel: Documentation/networking/pktgen.rst
 
 Helper include files
 ====================