Create metric card for Frugal Score
_The documentation is not available anymore as the PR was closed or merged._
Proposing metric card for Frugal Score.

@albertvillanova or @lhoestq  -- there are certain aspects that I'm not 100% sure on (such as how exactly the distillation between BertScore and FrugalScore is done) -- so if you find that something isn't clear, please let me know!