Change examples to use AppExit

[Freyja-moth]

Describe the issue

Currently all examples on the website return () instead of AppExit, which is a pattern we should probably discourage.

[https://bevy.org/learn/quick-start/getting-started/ecs/]

fn main() {
    App::new().add_systems(Update, hello_world).run();
}

Proposed Solution

Change them to return AppExit

fn main() -> AppExit {
    App::new().add_systems(Update, hello_world).run()
}

[alice-i-cecile]

I'm not convinced that this is helpful for most applications, or worth distracting users to teach early.