My documentation problem was largely fixed by using Nixos. The actual OS instances are self-documenting that way.

As far as the documentation for the network setup itself goes, a simple wiki does the rest of the trick for me.

All you need is a local smtp server that relays to another, "real" smtp server. I have a postfix that is configured to use the Proton smtp servers (before that it was Googles). No issues on Telenet.