2d rotations are cumbersome

## How can Bevy's documentation be improved?

The docs for `Vec2`, `Dir2`, `Rot2`, and `Quat` are poorly cross-linked/associated. This means it is quite a headache for new users of these APIs to figure out how to do conversions, and some of the conversions seem needlessly convoluted.

Here is the incantation to get `Quat` from `Dir2` in 2d space:
```rust
let rotation = Quat::from_rotation_z(direction.rotation_from_x().as_radians());
```

Here is some code to place an entity at a random radial position around another entity:
```rust
let direction = rng.gen_range((0.)..TAU);
let mut point_transform = Transform::from_translation(Vec3::default().with_x(spawn_radius));
point_transform.translate_around(Vec3::default(), Quat::from_rotation_z(direction));
```

Here I'm getting the `Aabb` zone for doing 2d projectile intersection tests. Yes, `Aabb2d` panics randomly if you don't correct by `PI - rotation`...
```rust
let mut rotation = Quat::default().angle_between(zone_transform.rotation.normalize());
if rotation > PI / 2. {
rotation = PI - rotation;
}
rotation = rotation.clamp(0., PI / 2.);
let entity_aabb = zone_aabb
.get_2d_from_vec(Vec2::default())
.transformed_by(zone_transform.translation.truncate(), rotation);
let entity_aabb = AabbCast2d::new(
entity_aabb,
Vec2::default(),
Dir2::new(last_pos - zone_transform.translation.truncate())
.unwrap_or(Dir2::new_unchecked(Vec2::default().with_x(1.))),
(zone_transform.translation.truncate() - last_pos).length(),
);
```

Start by reading the documentation for Vec2, Dir2, Rot2, and Quat, using the reported conversion and rotation examples as the current user experience. Map where these APIs and Aabb2d are documented, then define completion as clearer cross-links and less convoluted guidance for 2D rotation conversions.

Written by the indexing model from the issue text.