docs: networking: convert PLIP.txt to ReST

- add SPDX header;
- 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>

drivers/net/plip/Kconfig

@@ -21,7 +21,7 @@ config PLIP
 	  bits at a time (mode 0) or with special PLIP cables, to be used on
 	  bidirectional parallel ports only, which can transmit 8 bits at a
 	  time (mode 1); you can find the wiring of these cables in
-	  <file:Documentation/networking/PLIP.txt>.  The cables can be up to
+	  <file:Documentation/networking/plip.rst>.  The cables can be up to
 	  15m long.  Mode 0 works also if one of the machines runs DOS/Windows
 	  and has some PLIP software installed, e.g. the Crynwr PLIP packet
 	  driver (<http://oak.oakland.edu/simtel.net/msdos/pktdrvr-pre.html>)