Update api-overlay.mdx (#6212)

Most used cases for overlay are Alert and Toast which requires to see behind the overlay

website/api/api-overlay.mdx

   component: {
     name: 'example.Overlay',
     options: {
+      layout: {
+            componentBackgroundColor: 'transparent',
+          },
       overlay: {
         interceptTouchOutside: true
       }