Split the Elements notebooks into pieces

[jlstevens]

This issue summarises a discussion I've have with @jbednar and @philippjfr

We would like to get rid of the existing elements notebooks and have an Elements section in the website sidebar which expands with a list of all the elements. At the top of this list would be a navigation page showing a grid of thumbnails for all the different backends.

Each element notebook would then show how that element is constructed across backends. This proposal has the following advantages:

1. Makes it easier to add new elements (no need to update the whole notebook)
2. Makes it easier to test elements (identify which element went wrong given a failure)
3. Makes it easier to update those notebook tests
4. Make it easier to compare elements across backends.

To generate thumbnails, we would use a notebook archive. These thumbnails would be shown in a grid and there could be buttons to toggle the thumbnails between backends (unsupported elements would be greyed out appropriately).

This could all be organized in doc in an Elements subdirectory. For instance, I can imagine the following file paths:

doc/Elements/Index.html
doc/Elements/Image.ipynb
doc/Elements/thumbnails/mpl/Image.png
doc/Elements/thumbnails/bokeh/Image.png

[jbednar]

Thanks for the summary!

[philippjfr]

I still need to do Surface and Trisurface otherwise this is ready.

[jlstevens]

I think we also decided to do something similar for the container notebooks in the end.

[philippjfr]

Added Surface and Trisurface today, closing.

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