docs: Adds Customize Menu section Fiddle example (#20468)

docs/fiddles/menus/customize-menus/renderer.js

@@ -0,0 +1,8 @@
+const { ipcRenderer } = require('electron')
+
+// Tell main process to show the menu when demo button is clicked
+const contextMenuBtn = document.getElementById('context-menu')
+
+contextMenuBtn.addEventListener('click', () => {
+  ipcRenderer.send('show-context-menu')
+})