fix myst config

[OriolAbril]

Description

Fix myst configuration to work with latest versions.

Checklist

• [x] Does the PR follow official PR format?
• [x] Is the documentation numpydoc compliant?
• [ ] Is the fix listed in the Documentation section of the changelog?

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:books: Documentation preview :books:: https://arviz--2292.org.readthedocs.build/en/2292/