[DOCS-4039] Add CSI doc

[maycmlee]

What does this PR do?

• Adds new CSI doc and new nav item

Motivation

DOCS-4039

Additional Notes

Merge during keynote

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Reviewer checklist

• [ ] Review the changed files.
• [ ] Review the URLs listed in the Preview section.
• [ ] Check images for PII
• [ ] Review any mentions of "Contact Datadog support" for internal support documentation.

[github-actions[bot]]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/may/csi/security_platform/cloud_siem/cloud_security_investigator

Modified Files