Added on_unimplemented Diagnostic for IntoObserverSystem (#14840)

# Objective

- Fixes #14658.

## Solution

- Added `on_unimplemented` Diagnostic for `IntoObserverSystem` calling
out argument ordering in a `note`
- Added an example to the documentation on `App::observe` to provide
some explanation to users.

## Testing

- Ran CI locally
- Deliberately introduced a parameter order error in the
`ecs/observers.rs` example as a test.

---

## Showcase

<details>
  <summary>Error Before</summary>

```
error[E0277]: the trait bound `{closure@examples/ecs/observers.rs:19:13: 22:37}: IntoObserverSystem<_, _, _>` is not satisfied
   --> examples/ecs/observers.rs:19:13
    |
18  |           .observe(
    |            ------- required by a bound introduced by this call
19  | /             |mines: Query<&Mine>,
20  | |             trigger: Trigger<ExplodeMines>,
21  | |             index: Res<SpatialIndex>,
22  | |              mut commands: Commands| {
...   |
34  | |                 }
35  | |             },
    | |_____________^ the trait `bevy::prelude::IntoSystem<bevy::prelude::Trigger<'static, _, _>, (), _>` is not implemented for closure `{closure@examples/ecs/observers.rs:19:13: 22:37}`, which is required by `{closure@examples/ecs/observers.rs:19:13: 22:37}: IntoObserverSystem<_, _, _>`
    |
    = note: required for `{closure@examples/ecs/observers.rs:19:13: 22:37}` to implement `IntoObserverSystem<_, _, _>`
note: required by a bound in `bevy::prelude::App::observe`
   --> C:\Users\Zac\Documents\GitHub\bevy\crates\bevy_app\src\app.rs:995:24
    |
993 |     pub fn observe<E: Event, B: Bundle, M>(
    |            ------- required by a bound in this associated function
994 |         &mut self,
995 |         observer: impl IntoObserverSystem<E, B, M>,
    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `App::observe`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `bevy` (example "observers") due to 1 previous error
```

</details>

<details>
  <summary>Error After</summary>

```
error[E0277]: `{closure@examples/ecs/observers.rs:19:13: 22:37}` cannot become an `ObserverSystem`
    --> examples/ecs/observers.rs:19:13
     |
18   |           .observe(
     |            ------- required by a bound introduced by this call
19   | /             |mines: Query<&Mine>,
20   | |             trigger: Trigger<ExplodeMines>,
21   | |             index: Res<SpatialIndex>,
22   | |              mut commands: Commands| {
...    |
34   | |                 }
35   | |             },
     | |_____________^ the trait `IntoObserverSystem` is not implemented
     |
     = help: the trait `bevy::prelude::IntoSystem<bevy::prelude::Trigger<'static, _, _>, (), _>` is not implemented for closure `{closure@examples/ecs/observers.rs:19:13: 22:37}`, which is required by `{closure@examples/ecs/observers.rs:19:13: 22:37}: IntoObserverSystem<_, _, _>`
     = note: for function `ObserverSystem`s, ensure the first argument is a `Trigger<T>` and any subsequent ones are `SystemParam`
     = note: required for `{closure@examples/ecs/observers.rs:19:13: 22:37}` to implement `IntoObserverSystem<_, _, _>`
note: required by a bound in `bevy::prelude::App::observe`
    --> C:\Users\Zac\Documents\GitHub\bevy\crates\bevy_app\src\app.rs:1025:24
     |
1023 |     pub fn observe<E: Event, B: Bundle, M>(
     |            ------- required by a bound in this associated function
1024 |         &mut self,
1025 |         observer: impl IntoObserverSystem<E, B, M>,
     |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `App::observe`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `bevy` (example "observers") due to 1 previous error
```

</details>

crates/bevy_app/src/app.rs

@@ -990,6 +990,36 @@ impl App {
     }
 
     /// Spawns an [`Observer`] entity, which will watch for and respond to the given event.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// # use bevy_app::prelude::*;
+    /// # use bevy_ecs::prelude::*;
+    /// # use bevy_utils::default;
+    /// #
+    /// # let mut app = App::new();
+    /// #
+    /// # #[derive(Event)]
+    /// # struct Party {
+    /// #   friends_allowed: bool,
+    /// # };
+    /// #
+    /// # #[derive(Event)]
+    /// # struct Invite;
+    /// #
+    /// # #[derive(Component)]
+    /// # struct Friend;
+    /// #
+    /// // An observer system can be any system where the first parameter is a trigger
+    /// app.observe(|trigger: Trigger<Party>, friends: Query<Entity, With<Friend>>, mut commands: Commands| {
+    ///     if trigger.event().friends_allowed {
+    ///         for friend in friends.iter() {
+    ///             commands.trigger_targets(Invite, friend);
+    ///         }
+    ///     }
+    /// });
+    /// ```
     pub fn observe<E: Event, B: Bundle, M>(
         &mut self,
         observer: impl IntoObserverSystem<E, B, M>,

crates/bevy_ecs/src/system/observer_system.rs

@@ -25,6 +25,11 @@ impl<
 }
 
 /// Implemented for systems that convert into [`ObserverSystem`].
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` cannot become an `ObserverSystem`",
+    label = "the trait `IntoObserverSystem` is not implemented",
+    note = "for function `ObserverSystem`s, ensure the first argument is a `Trigger<T>` and any subsequent ones are `SystemParam`"
+)]
 pub trait IntoObserverSystem<E: 'static, B: Bundle, M, Out = ()>: Send + 'static {
     /// The type of [`System`] that this instance converts into.
     type System: ObserverSystem<E, B, Out>;