Use datatree from xarray and update example to latest einstats

[OriolAbril]

Description

Fix an example that was broken with the changes in xarray-einstats
to support preliz and the new scipy distributions
and have to/from_datatree methods use xarray instead of xarray-datatree discontinued library.

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

---

📚 Documentation preview 📚: https://arviz--2458.org.readthedocs.build/en/2458/

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 86.81%. Comparing base (896da8d) to head (89d35ee).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
arviz/data/inference_data.py	50.00%	2 Missing ⚠️

❌ Your patch status has failed because the patch coverage (50.00%) is below the target coverage (75.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2458   +/-   ##
=======================================
  Coverage   86.81%   86.81%           
=======================================
  Files         124      124           
  Lines       12972    12972           
=======================================
  Hits        11262    11262           
  Misses       1710     1710           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.