Add docs on how to configure a hosted Grafana instances

What does this Merge Request do?

Follow-up from !1124 (comment 324683156).

This updates the how-to on hooking up a GitLab project to Grafana to include instructions on how to create a hosted Grafana instance, as it's note very intuitive. It must be done after creating an account & isn't mentioned on the Grafana landing pages. So, docs will help others.

Merge Request checklist

• [-] Tests added for new functionality. If not, please raise Issue to follow-up.
• [-] This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• [-] gdk doctor test added, if needed.