Change Router::with_state and impl Service for Router<()> (#1552)

* Implement `Service` for `Router<(), B>`

* wip

* wip

* fix some tests

* fix examples

* fix doc tests

* clean up docs

* changelog

* fix

* also call `with_state` when converting `MethodRouter` into a `MakeService`

* suggestions from review

axum-macros/tests/from_request/pass/state_explicit.rs

@@ -6,7 +6,7 @@ use axum::{
 };
 
 fn main() {
-    let _: axum::routing::RouterService = Router::new()
+    let _: axum::Router = Router::new()
         .route("/b", get(|_: Extractor| async {}))
         .with_state(AppState::default());
 }