Add @alt.theme.register to API Reference

[dangotbanned]

@mattijn

I searched for register_theme within the altair documentation page, but nothing was returned altair-viz.github.io/search.html?q=register_theme. A brief description to the documentation page would help the average user a lot here.

Well spotted!
I wasn't aware this hadn't been included in the API reference yet, but will open a new issue/PR to address it - since that is orthogonal to this PR.

Originally posted by @dangotbanned in #3536 (comment)

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I added this to alt.__init__.__all__ in #3526, but forgot to make sure it was picked up by https://github.com/vega/altair/blob/fa420f36e0af8043d7197cda39a2834f7659cd68/tools/generate_api_docs.py