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>

Documentation/admin-guide/kernel-parameters.txt

@@ -4910,7 +4910,7 @@
 			Set the number of tcp_metrics_hash slots.
 			Default value is 8192 or 16384 depending on total
 			ram pages. This is used to specify the TCP metrics
-			cache size. See Documentation/networking/ip-sysctl.txt
+			cache size. See Documentation/networking/ip-sysctl.rst
 			"tcp_no_metrics_save" section for more details.
 
 	tdfx=		[HW,DRM]