Imbalance review (#19)

* Phrasing in intro

* Phrasing / placeholder for exercise

* Fix matplotlib import for jupyterlite

* Improve phrasing for the strictly proper scoring rule intro

* Phrasing in the decision thresholding section

* More phrasing improvements

* More phrasing improvements

* Improve phrasing and explanations for degenerate predictions

* Use a different variable name for the resampling model + fix placeholders

* Reframe the practical setting for the decision cutoff, fix one more placeholder and add conclusion

* Grammar

* Z -> z

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Co-authored-by: Guillaume Lemaitre <guillaume@probabl.ai>

Author: ogrisel