stale: how to access Prometheus data via Grafana

[jnywong]

Context

The pages

• https://docs.2i2c.org/admin/monitoring/prometheus-grafana-access/
• https://docs.2i2c.org/admin/monitoring/prometheus-grafana-cost/

are now stale ever since we migrated from ingress-nginx to nginx-ingress, since access to Prometheus data requires basic-auth credentials

• Figure out how to fix certificate renewals of nginx-ingress clusters infrastructure#8063.

Definition of done

• update these pages