Specify build formats in .readthedocs.yaml

[chrysle]

Fixes #1887

@webknjaz Another solution would be the approach in IQSS/dataverse#3542, if you want to keep building EPUB.

Contributor checklist

• Provided the tests for the changes.
• Assure PR title is short, clear, and good to be included in the user-oriented changelog

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• Assure one of these labels is present: backwards incompatible, feature, enhancement, deprecation, bug, dependency, docs or skip-changelog as they determine changelog listing.
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[webknjaz]

if you want to keep building EPUB.

There's also latex. But I don't think we ever cared about targeting those. So I tend to just build html everywhere..

[chrysle]

There's also latex.

Right, that's strangely not listed in the relevant section.

[webknjaz]

Right, that's strangely not listed in the relevant section.

That's a step in making a pdf.