Widget Factory: remove duplicated page

Fixes gh-335

Author: sigorilla

order.json

@@ -75,8 +75,7 @@
     "plugins": [
       "finding-evaluating-plugins",
       "basic-plugin-creation",
-      "advanced-plugin-concepts",
-      "stateful-plugins-with-widget-factory"
+      "advanced-plugin-concepts"
     ]
   },
   {

page/plugins.md

@@ -14,3 +14,5 @@ A jQuery plugin is simply a new method that we use to extend jQuery's prototype
 The idea of a plugin is to do something with a collection of elements. You could consider each method that comes with the jQuery core a plugin, like `.fadeOut()` or `.addClass()`.
 
 You can make your own plugins and use them privately in your code or you can release them into the wild. There are thousands of jQuery plugins available online. The barrier to creating a plugin of your own is so low that you'll want to do it straight away!
+
+For details on writing stateful plugins, see [How To Use the Widget Factory](/jquery-ui/widget-factory/how-to-use-the-widget-factory/).