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>
diff --git a/net/Kconfig b/net/Kconfig
index 8b1f858..c5ba2d1 100644
--- a/net/Kconfig
+++ b/net/Kconfig
@@ -344,7 +344,7 @@
 	  what was just said, you don't need it: say N.
 
 	  Documentation on how to use the packet generator can be found
-	  at <file:Documentation/networking/pktgen.txt>.
+	  at <file:Documentation/networking/pktgen.rst>.
 
 	  To compile this code as a module, choose M here: the
 	  module will be called pktgen.