Updates to interrupted time series notebook

[drbenvincent]

• Closes Expand docs: more info required about aggregating the causal impact over time #498
• Adds more thorough explanation of how to interpret the estimated causal impact in a interrupted time series notebook

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📚 Documentation preview 📚: https://causalpy--501.org.readthedocs.build/en/501/

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.13%. Comparing base (0449370) to head (a4e0a95).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #501   +/-   ##
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  Coverage   95.13%   95.13%           
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  Files          28       28           
  Lines        2384     2384           
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  Hits         2268     2268           
  Misses        116      116           

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[drbenvincent]

Tests now passing - there was a typo in the notebook