prepare 0.20.0 release

[OriolAbril]

Description

Checklist

• Follows official PR format
• Includes a sample plot to visually illustrate the changes (only for plot-related functions)
• New features are properly documented (with an example if appropriate)?
• Includes new or updated tests to cover the new feature
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

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📚 Documentation preview 📚: https://arviz--2389.org.readthedocs.build/en/2389/

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.81%. Comparing base (7e51ec9) to head (216bf56).
Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2389   +/-   ##
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  Coverage   86.81%   86.81%           
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  Files         124      124           
  Lines       12882    12882           
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  Hits        11183    11183           
  Misses       1699     1699           

Flag	Coverage Δ
	86.81% <100.00%> (ø)

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[aloctavodia]

LGTM, except I am not able to see what are the actual changes on the notebook.

[OriolAbril]

No changes, only checked it ran with the newest versions of everything