Document frontend redirect

[rdubrock]

Customers want to link to check dashboards from external sources (like alerts, their own custom dashboards, etc). This is difficult because the path for a check's dashboards includes it's ID (/a/grafana-synthetic-monitoring-app/<the check id>/dashboard, which isn't available in metrics. To handle this eventuality, there is a frontend redirect that will match a job/instance combo to a check ID and redirect to the appropriate dashboard: /a/grafana-synthetic-monitoring-app/scene?var-job=<the job name>&var-instance=<the target>. People don't know this redirect exists, though, so we should make it known.

This will solve the issue for now, but ultimately I think it would probably be nicer to add the check ID to the check_info metric so that it can be used in data links more easily.