docs(gateway): improve cluster deployment guide and refactor deploy scripts

Documentation improvements based on hands-on 2-node cluster deployment:
- Add CVM deployment section (section 3) with env vars, deploy steps, and
  port planning for single-host multi-node setups
- Add app ingress and port routing section (section 4) explaining SNI format
- Add test app deployment section (section 5) with end-to-end verification
- Add resource sizing and networking modes overview
- Fix subnet scheme to use SUBNET_INDEX 0-based allocation matching deploy
  script behavior
- Add SUBNET_INDEX mapping table and cluster sync/peer discovery docs
- Add image version note about sync enable logic differences
- Fix gateway-1 config: first node should have empty bootnode
- Expand verification section with health check criteria
- Add Cloudflare DNS:Edit permission note

Script changes:
- Extract bootstrap logic from deploy-to-vmm.sh into standalone
  bootstrap-cluster.sh for independent admin configuration
- Simplify deploy-to-vmm.sh to focus on CVM deployment only

Author: kvinwang