Clarify that ISP with NAT cannot do port forwarding

[Cerlancism]

Besides issuing dynamic IP address, more and more ISPs are also using NAT, which prevents a trivial port forwarding by configurating the home gateway.

I suggest to integrate the PIA article (https://support.storj.io/hc/en-us/articles/360026892971-Running-a-V3-Storage-Node-with-PIA-VPN-) to the networking section of the prerequisites (https://documentation.storj.io/before-you-begin/prerequisites#internet-connection) and port forwarding page (https://documentation.storj.io/dependencies/port-forwarding).

[tempestb]

There are alternative options, such as using VPS and proxy port mapping, as well as alternative VPN tools. To your point, we could include an explanation of NAT challenges and point to different documents on setting up one of the alternatives. Starting with PIA, and I can put together a document on VPS mapping.