Make docs home page friendlier

[ianhi]

Problem

Currently the landing page of the docs is not super inviting. https://ipywidgets.readthedocs.io/en/latest/index.html#

1. There is no explanatory or welcome text
2. The TOC in the sidebar is totally collapsed:
   

Proposed Solution

1. Add some friendly opening text.
   • e.g. "Ipywidgets is a library that ...." - "If you want to learn ___ go to <link>, if you want to ___ go to <link 2>"
2. Add a gif of some awesome widget interactions
   • and/or add some active widgets to the

Additional Context:

Two widget libraries that could serve as inspiration:
https://ipycytoscape.readthedocs.io/en/latest/
https://ipycanvas.readthedocs.io/en/latest/

[willingc]

PR #3104 is a first pass at this task using the suggested projects as inspiration. Thanks @ianhi.