docs: networking: convert secid.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 <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/index.rst

@@ -101,6 +101,7 @@ Contents:
    regulatory
    rxrpc
    sctp
+   secid
 
 .. only::  subproject and html
 

Documentation/networking/secid.txt → Documentation/networking/secid.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+=================
+LSM/SeLinux secid
+=================
+
 flowi structure:
 
 The secid member in the flow structure is used in LSMs (e.g. SELinux) to indicate