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>

net/ipv4/Kconfig

@@ -49,7 +49,7 @@ config IP_ADVANCED_ROUTER
 
 	  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.
 

net/ipv4/icmp.c

@@ -853,7 +853,7 @@ static bool icmp_unreach(struct sk_buff *skb)
 		case ICMP_FRAG_NEEDED:
 			/* for documentation of the ip_no_pmtu_disc
 			 * values please see
-			 * Documentation/networking/ip-sysctl.txt
+			 * Documentation/networking/ip-sysctl.rst
 			 */
 			switch (net->ipv4.sysctl_ip_no_pmtu_disc) {
 			default: