WIP: Document global known-good indicators

[daniel-trinh]

Summary

• Add a customer-facing Cloud SIEM page describing Datadog-managed global known-good indicators.
• Add an explicit log-processing flow from the known-good check through threat-intelligence enrichment.
• Add the page to the Ingest and Enrich navigation in the current hugo/ documentation layout.

Validation

• git diff --check origin/master...HEAD
• Not run: Vale and the local Hugo binary are unavailable in this checkout.

Additional notes

• Rebases onto master at aa5f058608.
• Draft for documentation-team review; no Docs Jira ticket has been assigned.

[github-actions]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/daniel.trinh/global-known-good-docs/security/cloud_siem/ingest_and_enrich/global_known_good_indicators