docs: add security model document.

[wy471x]

related to #6405

All changes

• Add Apache ShenYu Security Model document defining trust boundaries, deployment constraints, authentication model, RBAC scope, WebSocket sync channel security, control-plane/data-plane separation, and vulnerability scope
• Backport the document to version 2.7.1 docs with a corresponding Chinese (zh) translation
• Helps operators, security researchers, and the ASF Security team triage vulnerability reports against a clearly defined threat model