docs: networking: convert atm.txt to ReST

There isn't much to be done here. Just:

- add SPDX header;
- add a document title.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/atm.txt → Documentation/networking/atm.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+===
+ATM
+===
+
 In order to use anything but the most primitive functions of ATM,
 several user-mode programs are required to assist the kernel. These
 programs and related material can be found via the ATM on Linux Web

Documentation/networking/index.rst

@@ -41,6 +41,7 @@ Contents:
    altera_tse
    arcnet-hardware
    arcnet
+   atm
 
 .. only::  subproject and html
 

net/atm/Kconfig

@@ -16,7 +16,7 @@ config ATM
 	  of your ATM card below.
 
 	  Note that you need a set of user-space programs to actually make use
-	  of ATM.  See the file <file:Documentation/networking/atm.txt> for
+	  of ATM.  See the file <file:Documentation/networking/atm.rst> for
 	  further details.
 
 config ATM_CLIP