Add information about the network discovery and UPnP

We should describe the IP discovery and port forwarding, how it is working and the pitfalls to avoid

No file, test, or entry point is named. Start by locating the documentation area covering network discovery or port forwarding, then research the existing IP discovery and UPnP behavior and document how they work and which pitfalls to avoid; done means those topics are clearly covered in the relevant documentation.

Written by the indexing model from the issue text.