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, its per-request random fallback, recovery transitions, severity mapping, and label limits
• explicitly warn that a random fallback cannot correlate later repeat or recovery deliveries
• register Datadog in navigation and the integration document map
• keep setup instructions text-only because the screenshot could not be published through the approved CDN path

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
• executed both generated IIFE bundles and verified non-empty window.FlashDocsZh.Datadog and window.FlashDocsEn.Datadog

Test deployment gate

After merging into test, wait for Upload integration docs to OSS to succeed, then verify that both https://docs-cdn.flashcat.cloud/test/docs/zh.js and en.js contain the Datadog key before accepting the integration-center document display. No fc-saas-web change is required because datadog.alert is already mapped to that runtime key in both languages.

Related

• flashcatcloud/fc-event#2320