docs: networking: convert x25.txt to ReST

Not much to be done here:
- add SPDX header;
- add a document title;
- 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

···
    vrf
    vxlan
    x25-iface
+   x25
 
 .. only::  subproject and html
 

Documentation/networking/x25.txt Documentation/networking/x25.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+==================
 Linux X.25 Project
+==================
 
 As my third year dissertation at University I have taken it upon myself to
 write an X.25 implementation for Linux. My aim is to provide a complete X.25

net/x25/Kconfig

···
 	  You can read more about X.25 at <http://www.sangoma.com/tutorials/x25/> and
 	  <http://docwiki.cisco.com/wiki/X.25>.
 	  Information about X.25 for Linux is contained in the files
-	  <file:Documentation/networking/x25.txt> and
+	  <file:Documentation/networking/x25.rst> and
 	  <file:Documentation/networking/x25-iface.rst>.
 
 	  One connects to an X.25 network either with a dedicated network card