PUBDEV-8209: Adding varsplits() and adding examples for varsplits() & feature_frequencies() to pydocs

[hannah-tillman]

For: PUBDEV-8209

Updated the model_categories.rst file as well to reflect moved files. Had to change the anomaly detection varsplits() description list to separate paragraphs because it wouldn't print properly as a list. Please let me know if anything needs to be updated.