[Update]Understanding iptables (#7161)

* [Update]Understanding iptables

Updated the tables and the chains supported after validating:

```root@localhost:~# sudo iptables -t nat -L
Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination         

Chain INPUT (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination         

Chain POSTROUTING (policy ACCEPT)
target     prot opt source               destination     ```

* format the table

docs/guides/security/firewalls/what-is-iptables/index.md

@@ -23,7 +23,14 @@ This guide helps you understand iptables and explains what is iptables. It gets
 
 A table is a collection of chains that serves a particular function. The 3 main tables in iptables are the Filter, NAT, and Mangle tables.
 
-!["iptables table of tables](iptables-table-of-tables.png "iptables table of tables")
+| **Filter**        | **NAT**                  | **Mangle**                        |
+|-------------------|--------------------------|-----------------------------------|
+| INPUT chain       | INPUT chain              | INPUT chain                       |
+| FORWARD chain     | OUTPUT chain             | FORWARD chain                     |
+| OUTPUT chain      | PREROUTING chain         | OUTPUT chain                      |
+|                   | POSTROUTING chain        | PREROUTING chain                  |
+|                   |                          | POSTROUTING chain                 |
+
 
   - The **Filter Table** is used to control the flow of packets in and out of a system.
   - The **NAT Table** is used to redirect connections to other interfaces on the network.