docs: document public dashboard

service-catalog/rust-ci/public-dashboard.md

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+# Public dashboard
+
+Datadog can only be accessed by certain Rust teams.
+However certain data are useful to everyone, so the infra team created a
+[public dashboard](https://p.datadoghq.com/sb/3a172e20-e9e1-11ed-80e3-da7ad0900002-b5f7bb7e08b664a06b08527da85f7e30)
+to monitor the Rust CI.
+
+This dashboard is just a clone of the Datadog CI
+[dashboard](https://docs.datadoghq.com/continuous_integration/?site=us).
+
+These are some useful panels from the dashboard:
+
+- Pipeline duration: check how long the auto builds takes to run.
+- Top slowest jobs: check which jobs are taking the longest to run.
+- Change in median job duration: check what jobs are slowest than before. Useful
+  to detect regressions.
+- Top failed jobs: check which jobs are failing the most.