Expand CGN topic and add a command to help debug it

[Tommytrg]

In Open Your Ports! section we mention CGN but we could add more information and explain why is convenient to ask your ISP to disable it. We can also add a traceroute command to know if it's disabled or not.

[Tommytrg]

Instead of adding it to the documentation, we are going to write a Medium article explaining it and we can add the link to the documentation

[Tommytrg]

https://docs.witnet.io/node-operators/advanced-setups/docker-compose

https://docs.witnet.io/node-operators/docker-quick-start-guide