add log_prior group

[aloctavodia]

A log_prior group is needed for prior sensitivity checks as described here #2093

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📚 Documentation preview 📚: https://arviz--2312.org.readthedocs.build/en/2312/

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (0a91faf) 86.72% compared to head (dfd6da1) 86.72%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2312 +/- ## ======================================= Coverage 86.72% 86.72% ======================================= Files 122 122 Lines 12709 12709 ======================================= Hits 11022 11022 Misses 1687 1687 ```

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