RapierPhysicsPlugin requires generics

[Jondolf]

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

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:

RapierPhysicsPlugin::<()>::default()

or:

RapierPhysicsPlugin::<NoUserData>::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.

[Vrixyz]

I'm not too familiar with this part of the code, but I agree with the struggle.