Add documentation about how to customise scores

[nikosbosse]

787 changes how users interact with scoring rules. Instead of passing arguments to score() they are now expected to modify the scoring rules directly.

We have some documentation for how this works in the docs of score() and customise_metric(). In addition, we should add this to the scoringutils manuscript (#666) and the Getting-started vignette (#546).

[seabbs]

Note that I think this is a case of adding new documentation vs modifying out of date documentation as as far as I am aware the previous workflow wasn't discussed/documented anywhere.

[nikosbosse]

I've added some documentation in #874. Maybe this should become its own Vignette. That depends a bit on #832 and also on whether we create other sub lists such as metrics_wis() or something like that.

I'm moving this to 2.1. because we have some documentation now and creating a vignette can be pushed out to the next release.

[seabbs]

Agree