[docs] update documentation with AUC-mu, average precision

[jameslamb]

I was reading https://lightgbm.readthedocs.io/en/latest/Features.html today and realized that a list of metrics is maintained there. That list is missing some of the newer things that @btrotta has added (average precision, AUC-mu). This PR proposes adding them.

I'm also proposing something I think we should do more of...adding a label like new in v3.0.0. Since we only host one version of the documentation, based on master, I think these types of annotations are useful. This is a practice I really appreciate in the scikit-learn docs, for example.

For example, https://scikit-learn.org/stable/modules/generated/sklearn.compose.ColumnTransformer.html#sklearn-compose-columntransformer

[StrikerRUS]

Good idea of adding version info!

For other items next time we can use a special Sphinx directive versionadded. (We cannot use it here because this directive is paragraph-level.)

[jameslamb]

Good idea of adding version info!

For other items next time we can use a special Sphinx directive versionadded. (We cannot use it here because this directive is paragraph-level.)

oh cool! I did not know about that!

[github-actions]

This pull request has been automatically locked since there has not been any recent activity since it was closed. To start a new related discussion, open a new issue at https://github.com/microsoft/LightGBM/issues including a reference to this.