`RapierPhysicsPlugin` requires generics

Trying to add the plugin the way that most Bevy plugins are added fails:

```rust
fn main() {
App::new()
.add_plugins((DefaultPlugins, RapierPhysicsPlugin::default()))
.run();
}
```

It fails because Rust cannot infer the type for `PhysicsHooks`. Instead, you need to do something like this:

```rust
RapierPhysicsPlugin::<()>::default()
```

or:

```rust
RapierPhysicsPlugin::::default()
```

This isn't obvious for new users, and is especially confusing because the plugin has no documentation example (outside the website's general usage guides). The use of `NoUserData` or the concept of physics hooks isn't even explained on the website before the "Advanced collision-detection" section, even though the plugin initialization is essentially the first thing that a user has to do. It shouldn't be necessary to use generics here.

Additionally, limiting the user to a single `PhysicsHooks` type feels rather limiting and annoying from a composability standpoint. But that's another issue.