Context

After our main pull-request was merged into openai/chatgpt-retrieval-plugin ([Feature] Addition of MongoDB Atlas datastore) jib.adegunloye@mongodb.com pointed out that the link from the README to the setup.md that goes through creation of collections and search indexes in Atlas UI was broken.

I fixed the link, and also used the opportunity to make naming consistent.

This pull-request does not add any new features, but it makes consistent reference to MongoDB Atlas (via "mongodb-atlas" as the DATASTORE environment variable name, and mongodb_atlas in directory names. 

Links in documentation have been manually tested. Unit and Integration tests are passing in our Continuous Integration system.

Definition of done

Pull-Request is merged.

https://github.com/openai/chatgpt-retrieval-plugin/pull/435/

Pitfalls

What should the implementer watch out for? What are the risks?