docs: scaffold guide and API indexes

docs/ADMIN-GUIDE/security/README.md

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+# Security
+
+> **Status:** Scaffold only. Security pages are not migrated yet.
+
+This chapter will contain authentication, authorization, SSO, secrets, RBAC, and security-control guidance for administrators.
+
+## Planned pages
+
+- `sso-providers.md` — migrate and verify [`docs/SSO-PROVIDERS.md`](../../SSO-PROVIDERS.md).
+- `secrets.md` — document secret handling after source/configuration verification.
+- `rbac.md` — document roles and permissions from the canonical implementation.
+
+Do not treat the root SSO document as current guidance until the dynamic provider-discovery behavior and all provider claims have been reconciled.
+
+## Related
+
+- [`Administrator guide`](../README.md)
+- [`API documentation`](../../API/README.md)
+- [[DEVELOPER-GUIDE/architecture/README|Architecture index]]