Module-style doc strings for examples

[alice-i-cecile]

Every example should have a consistent, basic description of what it does.

IMO, the cleanest way to do this is to use the following style at the beginning of each example .rs file:

//! This example demonstrates how to do something really cool with Bevy
//! Check out all of the [`Structs`] and [`Traits`] it uses

use bevy::prelude::*;
use bevy::ecs::SomethingSpecial;

fn main(){
   App::new()
   .add_system(do_cool_things)
   .run()
}

If you would like to tackle this issue, please go through each example and add / move the description to this format.