docs: networking: convert tcp-thin.txt to ReST

Not much to be done here:

- add SPDX header;
- adjust identation, whitespaces and blank lines where needed;
- add to networking/index.rst.

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/networking/index.rst

···
    strparser
    switchdev
    tc-actions-env-rules
+   tcp-thin
 
 .. only::  subproject and html
 

Documentation/networking/ip-sysctl.rst

···
 	initiated. This improves retransmission latency for
 	non-aggressive thin streams, often found to be time-dependent.
 	For more information on thin streams, see
-	Documentation/networking/tcp-thin.txt
+	Documentation/networking/tcp-thin.rst
 
 	Default: 0
 

Documentation/networking/tcp-thin.txt Documentation/networking/tcp-thin.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+====================
 Thin-streams and TCP
 ====================
+
 A wide range of Internet-based services that use reliable transport
 protocols display what we call thin-stream properties. This means
 that the application sends data with such a low rate that the
···
 ==========
 More information on the modifications, as well as a wide range of
 experimental data can be found here:
+
 "Improving latency for interactive, thin-stream applications over
 reliable transport"
 http://simula.no/research/nd/publications/Simula.nd.477/simula_pdf_file