Document grafana prometheus integration

What does this Merge Request do?

This MR adds documentation about how to use the GDK to exercise functionality described in https://docs.gitlab.com/ee/user/project/integrations/prometheus.html#embedding-grafana-charts

Also: The Prometheus doc page was getting quite long, this MR creates a new subfolder and adds the additional instructions in a separate file.

Merge Request checklist

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