docs: networking: convert netfilter-sysctl.txt to ReST

Not much to be done here:

- add SPDX header;
- add a document title;
- add a chapter markup;
- mark tables as such;
- 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

@@ -83,6 +83,7 @@ Contents:
    netconsole
    netdev-features
    netdevices
+   netfilter-sysctl
 
 .. only::  subproject and html
 

Documentation/networking/netfilter-sysctl.rst

@@ -1,8 +1,15 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=========================
+Netfilter Sysfs variables
+=========================
+
 /proc/sys/net/netfilter/* Variables:
+====================================
 
 nf_log_all_netns - BOOLEAN
-	0 - disabled (default)
+	- 0 - disabled (default)
-	not 0 - enabled
+	- not 0 - enabled
 
 	By default, only init_net namespace can log packets into kernel log
 	with LOG target; this aims to prevent containers from flooding host