Add section in the Godot Rust book on how to use multiplayer api

[ValorZard]

The multiplayer API in godot is a little hard to get a handle on, so it would be nice if we could have a section in the rust book that explains how to use it with godot-rust/gdext.

[Bromeon]

Wrong repo, this is the website (the book has a separate one). I'll move it 🙂

Could you be a bit more concrete about the content you imagine?
Especially since you also opened godot-rust/gdext#903.