Document manual fullstack axum server

[ealmloff]

Dioxus fullstack supports integrating with a custom axum server, but we currently don't have any documentation about that on the docsite. We should add a page about integrating with an existing axum router that covers:

1. How to do SSR with an axum server
2. How to use server functions in an existing axum server
3. The folder structure the axum server expects
4. How to serve the static wasm + js assets with an axum server