Add runbook for troubleshooting StatusCake monitor errors

Add an initial runbook for how to handle common errors resulting in
StatusCake alerts

docs/runbooks/troubleshoot-statuscake-monitor.md

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+# Troubleshoot StatusCake alert
+
+## When to use
+
+When [StatusCake](https://www.statuscake.com/) raises an alert about a library
+site being down.
+
+## Prerequisites
+
+- Access to the used StatusCake account
+- Access to [Grafana](../platform-environments.md)
+- Access to [Lagoon UI](../platform-environments.md)
+
+## Procedure
+
+1. Determine whether the site is accessible by accessing the `/health` endpoint
+   for the site
+2. Note the response
+3. Log into [StatusCake](https://www.statuscake.com/)
+4. Locate the uptime test for the library site in question
+5. Find the downtime root cause for alert based on the time the alert.
+6. Click *Extra details*
+7. Note the error
+8. Log into Grafana
+9. Click *Explore*
+10. Add a label filter with the label *namespace* and value being the name of
+    the site being down.
+11. Add a label filter with the label *app* and value *php*
+12. Set the time range to include the start of the alert
+13. Note any errors
+
+## Known problems
+
+These are a list of known problems and how to address them.
+
+### DNS lookup errors
+
+#### Observations
+
+- StatusCake reports *Request timeout* and `EAI_AGAIN` additional data.
+
+#### Action
+
+You can ignore this error. StatusCake is experiencing DNS lookup issues. These
+are outside the scope of the platform.
+
+### Corrupt Drupal cache
+
+#### Observations
+
+- `/health` reports HTTP status code 500
+- Grafana logs contain PHP exceptions
+
+#### Action
+
+1. Log into Lagoon UI
+2. Locate the project in question
+3. Locate the environment in question
+4. Locate *Tasks*
+5. Run the task *Clear Drupal caches* for the environment in question
+6. Wait for the task to finish
+7. Verify that `/health` reports HTTP status code 200
+
+### Database connection errors
+
+#### Observations
+
+- `/health` reports HTTP status code 500
+- Grafana logs contain `PDOException: SQLSTATE[HY000]`
+
+#### Actions
+
+Do nothing. This is likely caused by a restart of the underlying database.
+Experience shows that it takes about 20 minutes for the restart to complete.
+
+Note that such errors will affect all sites running on the platform and will
+result in multiple alerts being raised.