Add migration guide

Author: MrGVSV

_release-content/migration-guides/bevy_reflect_type_data_callbacks.md

@@ -0,0 +1,53 @@
+---
+title: "Type data creation and registration changes"
+pull_requests: [24518]
+---
+
+Type data definition and registration have been reworked to enable `on_insert` and `on_register` registration callbacks.
+
+Type data is no longer automatically implemented on types implementing `Clone`.
+Instead, it must be manually implemented or derived:
+
+```rust
+// BEFORE
+#[derive(Clone)]
+struct ReflectSomeTypeData;
+
+// AFTER
+#[derive(TypeData)]
+struct ReflectSomeTypeData;
+
+// or manually:
+// impl TypeData for ReflectSomeTypeData {}
+```
+
+Additionally, type data can no longer be inserted directly onto a `&mut TypeRegistration`.
+Instead, it must be inserted during construction, with an owned `TypeRegistration`.
+This was done to ensure that registration callbacks weren't accidentally missed.
+
+```rust
+// BEFORE
+let mut registration = TypeRegistration::of::<MyType>();
+registration.register_type_data::<SomeTypeData, _>();
+
+// AFTER
+let registration = TypeRegistration::of::<MyType>()
+  .register_type_data::<SomeTypeData, _>();
+});
+```
+
+Methods for registering type data on the `TypeRegistry` remain unchanged.
+However, because of the new restrictions around inserting type data on `TypeRegistration`,
+instances of `TypeRegistry::get_mut` can be replaced with the new `TypeRegistry::registration_scope`
+in order to insert multiple type data without additional lookups.
+
+```rust
+// BEFORE
+let registration = registry.get_mut(TypeId::of::<MyType>());
+registration.register_type_data::<SomeTypeData, _>();
+
+// AFTER
+let registration = registry.registration_scope(TypeId::of::<MyType>(), |mut registration| {
+  registration.register_type_data::<SomeTypeData, _>();
+});
+```