docs: update Helicone observability docs (#9928)

## Background
We've updated how to integrate with Helicone, so the documentation needs
to be updated to use Helicone's new setup.

## Summary
I have updated existing documentation on how to integrate with Helicone
observability.

## Checklist

- [ ] Tests have been added / updated (for bug fixes / features)
- [x] Documentation has been added / updated (for bug fixes / features)
- [ ] A patch changeset for relevant packages has been added (for bug
fixes / features - run pnpm changeset in the project root)
- [x] I have reviewed this pull request (self-review)

Author: juliettech13