Add Addons customization docs

[ericholscher]

This is start for this work,
and mostly just pulls examples from the Mkdocs page over to the Addons docs.

I think there's more to do here with examples,
but I think this is a reasonable starting point.

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📚 Documentation previews 📚

• User's documentation (docs): https://docs--11888.org.readthedocs.build/en/11888/

• Developer's documentation (dev): https://dev--11888.org.readthedocs.build/en/11888/

[humitos]

I would not document the CSS variables yet since we are having a discussion about that in readthedocs/addons#473 and we are thinking about removing support for them for now while we explore how to it in a reliable way.