Add link to score definitions in summarise_score()

[jamesmbaazam]

Would be nice to see what each score means when you print out the output of the function. This could be a simple print method that adds a simple "see more details in https://epiforecasts.io/scoringutils/articles/scoring-rules.html" or an extra argument like "details = TRUE/FALSE" that prints out the definitions.

[seabbs]

I like the idea of the link to docs. It I think it might be a bit verbose to print actual definitions