Improve documentation for realistic full-stack apps

Any realistic web app will have things like database connections, or calling other services on the server side. Due to Dioxus "hijacking" the `main` function and sharing it with browser and backend code, it's not easy to see how one would create database connection pools or RPC clients. Any documentation from `axum` or `tokio` is not very useful, with the way the `main` is setup in Dioxus.

Second problem is how to pass these DB pools or RPC clients to Dioxus server functions, so that they can be used to fetch data from external services. This should be possible via Axum's State mechanism, but since we're not dealing with Axum directly, this needs to be properly documented with good examples.

Right now the only somewhat related example is the [auth example](https://github.com/dioxuslabs/dioxus/blob/main/packages/fullstack/examples/axum-auth/src/main.rs), but even that is not really documented at all.