docs(menu): storybook - include a disabled menu item in collapsible

Makes sure that a disabled menu item is represented in VRTs for the
Collapsible variant. Sets one of the existing menu items to disabled.

Author: jawinn

components/menu/stories/menu.stories.js

@@ -435,6 +435,7 @@ Collapsible.args = {
       label: "Social Media",
       iconName: "ShareAndroid",
       isCollapsible: true,
+      isDisabled: true,
       items: [
         { label: "Defaults to not visible within closed item" },
       ],