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.

[humitos]

Looks good. There are a few pending decisions we may want to decide before merging; but other than that, looks like a good first step.

[humitos]

@agjohnson mentioned these dd- where not necessary in readthedocs/addons#488 (comment), so I'm not sure if he is planning to remove them.

[ericholscher]

@humitos I can remove the CSS variable reference if we aren't ready to fully list them out? I'd like to get at least the custom event stuff unblocked here if possible.

[agjohnson]

This looks good. I think there are benefits to authoring this type of documentation in the addons docs, and not a whole lot of benefit to authoring in our application docs. Not important for now, but it's something we should consider more.

[agjohnson]

This is another example of documenting this in Addons docs being less work. We have the mock API data in the Addons repo that we keep up to date for testing, this would be better than replicating this data to this repo and it going out of date.

[ericholscher]

Yea, there's a bunch of room for improvement here, but getting something documented is more important than doing a bunch of infra work to publish the same stuff in another fashion.