Add the first doctests on CI

[rudymatela]

This PR adds the first doctests on CI.

• add first doctest
• activate doctests on CI
• ... add more doctests? 🤔
• add doctests in other files?
• automate "search" for doctests in all files (maybe in a later PR?)
• ...

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 50.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 91.08%. Comparing base (076e9d8) to head (2e930a5).

Files with missing lines	Patch %	Lines
src/calibrated_explanations/utils/helper.py	50.00%	4 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #61      +/-   ##
==========================================
- Coverage   91.21%   91.08%   -0.13%     
==========================================
  Files           9        9              
  Lines        2823     2828       +5     
==========================================
+ Hits         2575     2576       +1     
- Misses        248      252       +4     

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