The forthcoming metrics execute these functions via their constructor and thus introduce errors at `load_all()`. We could also instead rename the fairness files to be run last, e.g. `zzz-fair.R`.

This doesn't actually function independently as a commit, as I've linked out to the metrics made with this function in docs, but is otherwise able to stand on its own.

`demographic_parity()`, `equalized_odds()`, and `equal_opportunity()` are all special cases of `fairness_metrics()`.

now refers to the yardstick functions and `.post` values in docs.

also, moves `max_positive_rate_diff()` to the top of the file so that `fairness_metric()` doesn't error out on load.

gives `fairness_metric()` output an additional `metric_factory` class