Reimplement polynomial_regression.py (#8889)

* Reimplement polynomial_regression.py

Rename machine_learning/polymonial_regression.py to
machine_learning/polynomial_regression.py

Reimplement machine_learning/polynomial_regression.py using numpy
because the old original implementation was just a how-to on doing
polynomial regression using sklearn

Add detailed function documentation, doctests, and algorithm
explanation

* updating DIRECTORY.md

* Fix matrix formatting in docstrings

* Try to fix failing doctest

* Debugging failing doctest

* Fix failing doctest attempt 2

* Remove unnecessary return value descriptions in docstrings

* Readd placeholder doctest for main function

* Fix typo in algorithm description

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