Create a widget-examples repository for custom widget authors

[jtpio]

This is something that was discussed today during the Jupyter Widgets team meeting: jupyter-widgets/team-compass#1 (comment)

Problem

There is already good documentation on how to use widgets.

However there could be more resources to help developers create their own custom widgets.

Suggested Improvement

#2919 already helps with the new cookiecutter based tutorial, now available on the latest tag on RTD: https://ipywidgets.readthedocs.io/en/latest/examples/Widget%20Custom.html

@ianhi has been keeping some notes on custom widget development in this repo: https://github.com/ianhi/custom-ipywidget-howto
The idea would be to build upon that and make that information more visible.

Similar to the "JupyterLab Extension Examples" repository: https://github.com/jupyterlab/extension-examples

There could also be a "Jupyter Widgets Examples" repo at https://github.com/jupyter-widgets/widget-examples

[jtpio]

First steps towards this in ianhi/custom-ipywidget-howto#13