docs: add NetBird Network Routes (Subnet Router) guide

CT100 now routes 192.168.42.0/24 for the personal group, letting
Mac/iPhone reach LAN devices that don't run NetBird (OpenWrt,
Proxmox nodes, NAS, printers, cameras, etc).

- IP forwarding persisted via /etc/sysctl.d/99-netbird-route.conf
- Route created with masquerade=true, metric=9999
- access_control_groups restricts who can use the route
- Chose CT100 over CT124 to avoid the Tailscale kernel-mode
  iptables collision documented in peer-deployment-ops.md

Doc covers: concept, API + UI creation, verification, troubleshooting
(forwarding, masquerade, LXC caveats, Tailscale interference),
and security considerations for narrowing the route CIDR.

README.md

@@ -21,6 +21,7 @@
 | [client-troubleshooting.md](./client-troubleshooting.md) | 客戶端 P2P 失敗、bufferbloat、Force Relay 等坑 |
 | [peer-deployment-ops.md](./peer-deployment-ops.md) | 在 LXC 用 Docker 部署客戶端、改名、Mac daemon 重啟 |
 | [groups-and-policies.md](./groups-and-policies.md) | Groups 與 Access Control Policies 概念、設計、API 操作 |
+| [network-routes.md](./network-routes.md) | Subnet Router：讓遠端 peer 存取 LAN 上沒裝 NetBird 的裝置 |
 | [SUMMARY.md](./SUMMARY.md) | 部署摘要與疑難排解記錄 |
 
 ## 快速開始