Improve ML docs/tests

[MichaeINeumann]

Summary\n- improve k-means docstrings/type hints and add convergence doctests\n- refactor gradient descent helpers and add targeted unit tests\n- add typed linear regression docs/examples and new ML pytest module\n\n## Testing\n- .venv/bin/pre-commit run --files machine_learning/k_means_clust.py machine_learning/linear_regression.py machine_learning/gradient_descent.py machine_learning/test_machine_learning_algorithms.py\n- .venv/bin/python -m pytest machine_learning/test_machine_learning_algorithms.py\n\nFixes #13919

[algorithms-keeper]

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