UiDebugOptions migration guide (#21088)

# Objective
Fixes #21036

---------

Co-authored-by: Alice Cecile <[email protected]>

Author: janis-bhm

release-content/migration-guides/ui-debug-overlay.md

@@ -0,0 +1,20 @@
+---
+title: Move UI Debug Options from `bevy_ui` to `bevy_ui_render`
+pull_requests: [18703]
+---
+
+The `UiDebugOptions` resource used for controlling the UI Debug Overlay has been moved from the internal `bevy_ui` crate to the `bevy_ui_render` crate, and is now accessible from the prelude of `bevy_ui_render` and, as before, from the prelude of `bevy`:
+
+```rust
+// 0.16
+use bevy::prelude::*;
+// or
+use bevy::ui::UiDebugOptions;
+
+// 0.17
+use bevy::prelude::*;
+// or, if you are not using the full `bevy` crate:
+// use bevy_ui_render::prelude::*;
+
+let options = world.resource_mut::<UiDebugOptions>();
+```