docs: document STUN port change to 3479

NetBird's default STUN port 3478/udp conflicts with Headscale on
192.168.42.126, which DNAT'd it first. Reconfigure NetBird to use
3479/udp end-to-end (server config + docker-compose + OpenWrt DNAT).

Updates server config:
- /opt/netbird/config.yaml: stunPorts: [3479]
- /opt/netbird/docker-compose.yml: 3479:3479/udp

OpenWrt DNAT rule added separately (42_10 repo commit 9cc6141).

README.md

@@ -17,6 +17,7 @@
 | [netbird-selfhosted-setup.md](./netbird-selfhosted-setup.md) | 完整安裝指南（含原理說明） |
 | [npm-grpc-fix.md](./npm-grpc-fix.md) | 透過 Nginx Proxy Manager 對外公開時的 gRPC 修復 |
 | [setup-keys-and-pat.md](./setup-keys-and-pat.md) | Setup Keys 概念、PAT 用法、以及 API 無法改名的繞道做法 |
+| [stun-port-conflict.md](./stun-port-conflict.md) | STUN 3478 被 Headscale 佔用 → NetBird 改用 3479 的解法 |
 | [SUMMARY.md](./SUMMARY.md) | 部署摘要與疑難排解記錄 |
 
 ## 快速開始
@@ -51,6 +52,7 @@ netbird up --management-url http://192.168.42.127
 |------|-----|
 | 主機 | `192.168.42.127` |
 | 系統 | Ubuntu 24.04 LTS |
+| STUN port | **3479/udp** (非標準 — 3478 被 Headscale 佔用) |
 | 容器類型 | Proxmox LXC (非特權) |
 | Docker | 29.4.0 |
 | OCI Runtime | crun 1.20 |