Add crate-level missing_docs to bevy_render_2d

Author: hukasu

crates/bevy_render_2d/src/lib.rs

@@ -1,5 +1,7 @@
 //! Provides functionality for rendering in 2d
 
+#![expect(missing_docs, reason = "Not all docs are written yet, see #3492.")]
+
 pub mod material;
 pub mod mesh_pipeline;