PagerDuty notifications are documented as an Anti-Pattern

Reading through [Sending Change Events from your pipeline](https://buildkite.com/docs/integrations/pagerduty#sending-change-events-from-your-pipeline) it seems that I should be storing the Integration API Key in the pipeline. However, this is documented as an anti-pattern: [Anti-pattern: Storing secrets in your pipeline.yml](https://buildkite.com/docs/pipelines/secrets#anti-pattern-storing-secrets-in-your-pipeline-dot-yml) because that API key should be a secret.

Since this integration is tightly coupled to Buildkite, the documentation should be improved. These things are not clear:

- Is `pagerduty_change_event` going to be redacted?
- What is the recommended way of storing this secret in a static pipeline?

Moreover, I believe that there should be a warning somewhere in the documentation. Currently, people might just go ahead and store their secret in a pipeline ([ref](https://github.com/buildkite/docs/pull/858)) because it seems like it is the official recommendation.