Document bevy_math (#4591)

# Objective

- Part of #3492

## Solution

- Document the `bevy_math` crate and add the `#![warn(missing_docs)]` lint.

crates/bevy_math/src/lib.rs

@@ -1,5 +1,12 @@
-pub use glam::*;
+//! Provides math types and functionality for the Bevy game engine.
+//!
+//! The commonly used types are vectors like [`Vec2`] and [`Vec3`],
+//! matrices like [`Mat3`] and [`Mat4`] and orientation representations
+//! like [`Quat`].
 
+#![warn(missing_docs)]
+
+/// The `bevy_math` prelude.
 pub mod prelude {
     #[doc(hidden)]
     pub use crate::{
@@ -7,3 +14,5 @@ pub mod prelude {
         Vec2, Vec3, Vec4,
     };
 }
+
+pub use glam::*;