expose new features as preview submodule

[OriolAbril]

Description

Checklist

• Does the PR follow official PR format?
• Does the PR include new or updated tests to cover the new feature (using pytest fixture pattern)? No, I tested locally.
• Is the code style correct (follows pylint and black guidelines)?
• Is the new feature listed in the New features section of the changelog?

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📚 Documentation preview 📚: https://arviz--2361.org.readthedocs.build/en/2361/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.01%. Comparing base (2bd2570) to head (ab5acd8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2361      +/-   ##
==========================================
+ Coverage   87.00%   87.01%   +0.01%     
==========================================
  Files         123      124       +1     
  Lines       12775    12788      +13     
==========================================
+ Hits        11115    11128      +13     
  Misses       1660     1660              

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[ahartikainen]

Lgtm