Add example use of custom priors to the notebook docs

[drbenvincent]

Following #488, I'm updating docs to show examples of using the custom priors.

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📚 Documentation preview 📚: https://causalpy--533.org.readthedocs.build/en/533/

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.50%. Comparing base (7dea992) to head (f95a7d9).
⚠️ Report is 34 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #533   +/-   ##
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  Coverage   96.50%   96.50%           
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  Files          31       31           
  Lines        4173     4173           
=======================================
  Hits         4027     4027           
  Misses        146      146           

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