docs: add security architecture overview page (#634)

Add docs/security.mdx with the full auth stack as the lead section:
password auth, SSO (5 providers + generic OIDC), TOTP MFA, JWT sessions,
API tokens, RBAC (5 roles), audit logging, AES-256-GCM encryption,
rate limiting, and node-to-node authentication.

Includes tier availability matrix, permission matrix, production
hardening checklist, and vulnerability reporting section. Links to
dedicated feature pages for configuration details.

Add navigation entry in docs.json Reference group.

docs/docs.json

@@ -125,6 +125,7 @@
           {
             "group": "Reference",
             "pages": [
+              "security",
               "features/node-compatibility",
               "reference/settings",
               "reference/verifying-images",