Document requirement for ipywidgets for UI support

[kwlzn]

• Document requirements for ipywidgets installation in the Jupyter Notebook/Lab environment w/ linkages
• This should be as easy as e.g. pip install jupyterlab_widgets in JupyterLab 3.x but should be validated.

ref: https://ipywidgets.readthedocs.io/en/latest/user_install.html#installing-in-jupyterlab-3-0

N.B. Colab has ipywidgets support built-in - and most preinstalled notebook envs will, but for fresh installs this is a requirement.