[DOCS] grouped metrics lack documentation

[koaning]

Feels like a good thing to add.

[saurabhya]

Hey @koaning !! I would like to contribute for this issue. Can you guide me on where in particular documentation is required and how can I proceed with this?

[koaning]

@MBrouns do we still want to document the metrics we have listed here?

https://github.com/koaning/scikit-lego/blob/main/sklego/metrics.py

[koaning]

Note: this one might not be ideal to pick up during the code spring. These grouped metrics may be ported to another library.