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

- add SPDX header;
- adjust titles and chapters, adding proper markups;
- mark code blocks and literals as such;
- mark lists as such;
- mark tables as such;
- use footnote markup;
- adjust identation, whitespaces and blank lines;
- add to networking/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/Kconfig b/net/ipv4/Kconfig
index 25a8888..5da4733 100644
--- a/net/ipv4/Kconfig
+++ b/net/ipv4/Kconfig
@@ -49,7 +49,7 @@
 
 	  Note that some distributions enable it in startup scripts.
 	  For details about rp_filter strict and loose mode read
-	  <file:Documentation/networking/ip-sysctl.txt>.
+	  <file:Documentation/networking/ip-sysctl.rst>.
 
 	  If unsure, say N here.