docs: Build docs with ArviZExampleData v0.3

[sethaxen]

Description

This PR uses ArviZExampleData v0.3 in the docs. The main change is that the r2_score docstring example now uses the new anes dataset instead of the discontinued regression1d dataset. It also fixes an error in the r2_score documented return value.

Checklist

• PR follows official PR format.
• The docs change is listed in the Documentation section of the changelog.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.04%. Comparing base (b2e8fd1) to head (ac5d0b5).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #86   +/-   ##
=======================================
  Coverage   97.04%   97.04%           
=======================================
  Files          16       16           
  Lines         845      845           
=======================================
  Hits          820      820           
  Misses         25       25           

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