ENH Add post-processing algorithm "Discrimination Aware Decision Tree Learning" by Kamiran et al.

[Hadyark]

This PR will solve #1024 and implements the Relabeling from the paper Discrimination Aware Decision Tree Learning by Kamiran et al.

• postprocessing technique code in fairlearn.postprocessing
• unit tests in test.unit.postprocessing
• descriptive API reference (directly in the docstring)
• example notebook
• a short user guide in docs.user_guide.mitigation.rst

[adrinjalali]

Thanks for the PR @Hadyark .

A few notes:

• Please make sure your lines are not too long, including in the documentation, lines should be <88 chars long.
• The main entry point should be a somewhat scikit-learn compatible estimator, added to the corresponding __init__.py file and exposed publicly.
• We follow numpydoc standard for docstrings, please have a look at the existing classes/methods and adapt the docstrings accordingly.
• Your .ipynb notebook file should be converted into a .py file with the same format as the other examples in examples folder.

I have approved the CI and merging with the latest main should render your documentation for us here.