Start Jupyter-Org/Widgets page with ipywidgets

The Widgets page in Jupyter-Org (https://jupyter.org/widgets) starts straight from an example of ipyleaflet. This may be misleading or confusing. It should start from what the Widgets fundamentally are: ipywidgets (https://ipywidgets.readthedocs.io).

Proposed action:
- [ ] Prepend sort description and example of ipywidgets