Fix and reenable notebook tests

[adri0]

• Refactor notebook tests to run all ipynb found in the docs folder.
• Allows for exclusion of currently breaking notebooks.
• Notebook tests will only run if the env variable TEST_NOTEBOOKS is set (which by default is not set).
• Reenable notebooks tests in Github test workflow (by setting TEST_NOTEBOOKS=1).
• Set a timeout (180 seconds) to which notebooks need to run.
• Refactor the notebook docs/tutorials/nb_automatic_best_num_features.ipynb to run faster.
• Fix compatibility issue on notebook docs/tutorials/nb_imputation_comparison.ipynb related to IterativeImputer.
• Fix breaking SHAP dependency.

[adri0]

Note to self: once this is merged create the following issues:

• Fix still failing notebooks.
• Reconsider enabling lightgbm tests?