Augment onboarding docs with Chroma demo

[lintool]

Extend onboarding doc to include shoving NFcorpus vectors) into Chroma - as a demonstration of how to use Chroma:
https://github.com/chroma-core/chroma

Why Chroma, because according to this: https://db-engines.com/en/ranking/vector+dbms
As of March 2024, it's the most popular open-source vector db.