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

@@ -1,9 +1,18 @@
-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::{
         BVec2, BVec3, BVec4, EulerRot, IVec2, IVec3, IVec4, Mat3, Mat4, Quat, UVec2, UVec3, UVec4,
         Vec2, Vec3, Vec4,
     };
 }
+
+pub use glam::*;