Perplexity Metric Card
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_3905). All of your documentation changes will be reflected on that endpoint. I'm wondering if we should add that perplexity can be used for analyzing datasets as well Otherwise, looks good! Good job, @emibaylor !
Add Perplexity metric card

Note that it is currently still missing the citation, but I plan to add it later today.