Documentation: Add more documentation + print method for default metrics

[nikosbosse]

There are now some default metrics stored as a list.
These need more documentation. Some ideas:

• Add them to a vignette
• Add documentation to score()
• Create an attribute with some explanations, make the stored objects belong to some class like scoringutils_metrics() and create a new print method for it.
• Create a new data.frame that lists all the metrics and store them as an object in scoringutils

[seabbs]

e stored objects belong to some class like scoringutils_metrics(

This seems like it could be a good idea?

[nikosbosse]

As mentioned in #415:

The default scoring rules for every forecast type are documented in the rules_*() function documentation which links to all individual functions. In addition, users should be able to find more information in the Vignette.

I think we don't necessarily need additional documentation beyond this. Creating a print method with additional information from the docs seems like something that's not really useful + hard to maintain. I'm therefore closing this for now, but please reopen if you disagree.