Elo-grad

Fix Python version syntax in docs.yml.

Add mkdocs documentation.

What's Changed

Breaking Changes

- `transform` method dropped from `EloEstimator`

Features

- `EloEstimator` is now `scikit-learn` compatible
- `EloEstimator` has a `track_rating_history` flag to track rating history via the 'rating_history` attribute
- `EloEstimator` has a `plot` method to plot rating history

Introduce `EloEstimator` to apply Elo rating system to a pandas DataFrame, producing expected scores and ratings.