Reorganize Widgets User guide

[Azaya89]

fixes #1641

• Create how-to section and notebook for widgets exploration
• Move panel widgets part to reference section
• Create Explanation section and notebook for groupby vs by vs color

[maximlt]

Adding this comment after going through the whole notebook. I like it but think it can be re-organized a little to be even better. How I see it is that there are three main use cases:

• Drilling down a dataset with widgets -> groupby
• Facetting a dataset -> by + subplots
• Color a dataset

The first two are pretty straightforward to explain. The last one is where things are a bit more subtle are users can use wither by or c/color but there are differences:

• by:
  • Creates containers (can be indexed and stuff)
  • Accept a list of dimensions
  • Generates an overlay -> If many overlays (many values in a dimension) then performance can be affected
  • Works only for categorical data (i.e. no nice colorbar for numerical data)
• color:
  • Single Element returned
  • Does not accept a list of dimensions
  • Can also be used for getting a nice colorbar

And I would document color before by as I think most users should probably use color?

[Azaya89]

Arranging them this way is a bit more 'poetic' as it kinda rolls off the tongue better: groupby -> by -> color. Putting color in-between both kinda breaks the flow IMO.