`AnimationPlayer` doesn't explain how it is created

## How can Bevy's documentation be improved?

[`bevy_animation::AnimationPlayer`](https://docs.rs/bevy_animation/latest/bevy_animation/struct.AnimationPlayer.html) doesn't explain:
- how you should create an animation player
- if you even *should* create an animation player manually
- how this interacts with `AnimationTransitions` and glTF scenes

When I was trying to spawn an entity with animations, I was following the animated fox example, but tried to simplify it. My code looked approximately like:
```rust
let mut anim_player = AnimationPlayer::default();
let mut transitions = AnimationTransitions::new();

transitions
.play(&mut anim_player, idle_animation, Duration::ZERO)
.repeat();

commands
.spawn((
SceneBundle {
scene: my_entity_gltf_scene.clone(),
..default()
},
anim_player,
transitions,
my_anim_graph.clone(),
));
```

However, this is wrong because here I'm creating my own `AnimationPlayer`, when I should actually be using the player created by the scene when it's instantiated. What I actually have to do is spawn the SceneBundle in one system, wait for the scene to be instantiated, and use a second system to use the automatically-created `AnimationPlayer` (I assume because the scene automatically populates it with some important data?). It's not really documented that this is specifically how you're meant to do this.