Add user-defined components to "What's new?" (#1562)

Author: alexandratran

docs/whats-new.md

@@ -11,6 +11,8 @@ of the [MetaMask developer page](https://metamask.io/developer/).
 
 ## September 2024
 
+- Documented [Snaps user-defined components](/snaps/features/custom-ui/user-defined-components).
+  ([#1557](https://github.com/MetaMask/metamask-docs/pull/1557))
 - Updated [Android SDK documentation](/wallet/how-to/use-sdk/mobile/android) with convenience
   methods and examples using coroutines.
   ([#1546](https://github.com/MetaMask/metamask-docs/pull/1546))