Enable jupyter-book=0.14 + sphinx book theme

[mmcky]

This PR upgrades jupyter-book to the latest released version which brings lots of improvements.

• test out references to figures that are generated by code (ref and numref roles)

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[mmcky]

@shlff and @jstac if you search for TEST ===> then you will see the example text where I use ref and numref to resolve to a figure produced by code.

https://6401920da8023c0759c96364--taupe-gaufre-c4e660.netlify.app/inequality.html#gini-coefficient-dynamics-for-us-data

@shlff some of the sizes may need to be set in the metadata to make them bigger.

[jstac]

Thanks @mmcky !

I think this will make it easier --- we can embed these nice features and then make the switch as required.