Merge pull request #2919 from jtpio/custom-widget

Add a cookiecutter based tutorial to build a custom widget

Author: ianhi

docs/source/examples/Index.ipynb

@@ -38,6 +38,7 @@
     "- [Widget List](Widget%20List.ipynb)  \n",
     "- [Widget Styling](Widget%20Styling.ipynb)\n",
     "- [Layout Templates](Layout%20Templates.ipynb)\n",
+    "- [Custom Widget Tutorial](Widget%20Custom.ipynb)\n",
     "- [Widget Asynchronous](Widget%20Asynchronous.ipynb): how to pause and listen in the kernel for widget changes in the frontend."
    ]
   },