docs(acp): add MetalLB host FRR S2 workaround

[changluyi]

Summary

• add English and Chinese S2 guidance for host systemd FRR conflicts with MetalLB FRR
• switch MetalLB to the native BGP backend to remove FRR-related speaker containers
• document prerequisites, verification, optional speaker-node isolation, and rollback

Validation

• git diff --check