Re-organize/simply notebook directories

[mwcraig]

The numbering and naming of the notebooks is getting pretty confusing. It would be nice to fix that.

[marimeireles]

Hey @mwcraig what do you propose?

[martinRenou]

I remember we made some changes for JupyterCon related to re-organizing folders and Notebooks https://github.com/jupytercon/2020-widgets-tutorial we should maybe port them back here

[mwcraig]

I didn't have specific layout in mind but I like the jupytercon one that Martin linked to.