Add headless toolbar note (#2734)

Add headless toolbar note

Author: edoversb

demos/toolbar/README.md

@@ -5,3 +5,7 @@ An example of how to add a custom button to the SuperDoc toolbar. This custom bu
 [We define the custom button in the `modules.toolbar.customButtons` option](https://github.com/superdoc-dev/superdoc/blob/main/demos/toolbar/src/main.js)
 
 The button's action is to insert a custom `catNode`. [The custom node and its Prosemirror click-handler plugin are defined in the same file](https://github.com/superdoc-dev/superdoc/blob/main/demos/toolbar/src/main.js).
+
+It is also possible to fully replace the toolbar with your own: [Headless Toolbar](https://docs.superdoc.dev/modules/toolbar/overview#learn-more)
+
+More customization options here: https://docs.superdoc.dev/modules/toolbar/overview