Add a section mentioning GetPath in bevy_reflect

The root doc for bevy_reflect doesn't mention `GetPath`. It's a fairly
useful featuree and I've seen people be surprised to learn it exists.

crates/bevy_reflect/src/lib.rs

@@ -224,6 +224,31 @@
 //!
 //! All primitives and simple types implement `FromReflect` by relying on their [`Default`] implementation.
 //!
+//! # Path navigation
+//!
+//! The [`GetPath`] trait allows accessing arbitrary nested fields of a [`Reflect`] type.
+//!
+//! Using [`GetPath`], it is possible to use a path strings to access a specific field
+//! of a reflected type.
+//!
+//! ```
+//! # use bevy_reflect::{Reflect, GetPath};
+//! #[derive(Reflect)]
+//! struct MyStruct {
+//!   value: Vec<Option<u32>>
+//! }
+//!
+//! let my_struct = MyStruct {
+//!   value: vec![None, None, Some(123)],
+//! };
+//! assert_eq!(
+//!   my_struct.path::<u32>(".value[2].0").unwrap(),
+//!   &123,
+//! );
+//! ```
+//!
+//! Check the [`GetPath`] documentation for more details.
+//!
 //! # Type Registration
 //!
 //! This crate also comes with a [`TypeRegistry`] that can be used to store and retrieve additional type metadata at runtime,