Network, ports, firewall reference page

[cloudjumpercat]

app/gateway/network-ports-firewall.md

See my note on the stub page. I don't know if this is one reference page, multiple pages, perhaps a landing page?

[lena-larionova]

This issue should just be a reference page. There should be a separate landing page for networking + how-tos/references some of the other pieces.

Original doc:
http://docs.konghq.com/gateway/latest/production/networking/firewall/
https://docs.konghq.com/gateway/latest/production/networking/default-ports/