Re-add the custom widget tutorial

[jtpio]

Follow-up to #3542

Re-add the custom widget tutorial which was removed in #3542

TODO

• Remove jupyter-packaging from the tutorial
• Wait for Update to ipywidgets 8 widget-ts-cookiecutter#119 to land
• Remove the mention to JupyterLab 2.x since JupyterLab 3 has been available for a while now

[github-actions]

👈 Launch a binder notebook on branch jtpio/ipywidgets/custom-widget-tutorial

[jtpio]

Kicking CI to trigger a new docs build, the previous run encountered the following issue: https://readthedocs.org/projects/ipywidgets/builds/18394828/

Command killed due to timeout or excessive memory consumption

[jasongrout]

@kellietay volunteered to review this

[kellietay]

Checks:

• fetched the PR. yarn run build is successful.
• sphinx build is successful. verified that index.html for the docs build looks good on local.