Onboarding docs - add Arctic embeddings

[lintool]

https://github.com/castorini/pyserini/blob/master/docs/experiments-nfcorpus.md#indexing

Add Arctic embedding model?

https://www.snowflake.com/blog/introducing-snowflake-arctic-embed-snowflakes-state-of-the-art-text-embedding-family-of-models/

Should be a one line change, but someone needs to test it out...