docs(on-call): add Datadog alert integration

[pijiang3]

Summary

• add Chinese and English Datadog alert integration guides
• document the approved default webhook payload and a separate custom_labels extension example
• explain $ALERT_CYCLE_KEY, recovery transitions, severity mapping, and label limits
• register Datadog in navigation and the integration document map
• add a visually reviewed Datadog Webhooks setup screenshot with no account or credential data

Validation

• jq empty docs.json
• npm --prefix integration-docs run build
• npm --prefix integration-docs run check
• built and verified 80 bilingual document keys on the current test branch

Related

• flashcatcloud/fc-event#2320