docs: consolidate state management docs in crate root (#3683)

Co-authored-by: Yann Simon <[email protected]>

axum-core/src/extract/from_ref.rs

@@ -1,13 +1,14 @@
 /// Used to do reference-to-value conversions thus not consuming the input value.
 ///
 /// This is mainly used with [`State`] to extract "substates" from a reference to main application
-/// state.
+/// state. See ["Sharing state with handlers"][sharing-state] for a worked example.
 ///
 /// See [`State`] for more details on how library authors should use this trait.
 ///
 /// This trait can be derived using `#[derive(FromRef)]`.
 ///
 /// [`State`]: https://docs.rs/axum/0.8/axum/extract/struct.State.html
+/// [sharing-state]: https://docs.rs/axum/0.8/axum/index.html#sharing-state-with-handlers
 // NOTE: This trait is defined in axum-core, even though it is mainly used with `State` which is
 // defined in axum. That allows crate authors to use it when implementing extractors.
 pub trait FromRef<T> {