DOCS-3949 - CSM

[kayayarai]

What does this PR do?

• Documents CSM Overview
• Restructures Security section nav
• Adds CSM category to Rules filters

Motivation

DOCS-3949

Additional Notes

---

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/kari/docs-3949-dash/security_platform/cloud_security_management/

Modified Files

• https://docs-staging.datadoghq.com/kari/docs-3949-dash/security_platform/cloud_security_management/guide/