docs: networking: convert sctp.txt to ReST

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

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/index.rst

@@ -100,6 +100,7 @@ Contents:
    rds
    regulatory
    rxrpc
+   sctp
 
 .. only::  subproject and html
 

Documentation/networking/sctp.rst

@@ -1,4 +1,8 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=================
 Linux Kernel SCTP
+=================
 
 This is the current BETA release of the Linux Kernel SCTP reference
 implementation.
@@ -19,7 +23,8 @@ of SCTP that is RFC 2960 compliant and provides an programming interface
 referred to as the  UDP-style API of the Sockets Extensions for SCTP, as
 proposed in IETF Internet-Drafts.
 
-Caveats:  
+Caveats
+=======
 
 - lksctp can be built as statically or as a module.  However, be aware that
   module removal of lksctp is not yet a safe activity.
@@ -29,7 +34,9 @@ implementation and testing lksctp on IPv4.
 
 
 For more information, please visit the lksctp project website:
+
    http://www.sf.net/projects/lksctp
 
 Or contact the lksctp developers through the mailing list:
+
    <linux-sctp@vger.kernel.org>

MAINTAINERS

@@ -14999,7 +14999,7 @@ M:	Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
 L:	linux-sctp@vger.kernel.org
 S:	Maintained
 W:	http://lksctp.sourceforge.net
-F:	Documentation/networking/sctp.txt
+F:	Documentation/networking/sctp.rst
 F:	include/linux/sctp.h
 F:	include/net/sctp/
 F:	include/uapi/linux/sctp.h