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).

netbird-selfhosted-setup.md

@@ -269,7 +269,7 @@ Client Request (:80)
 | 端口 | 協定 | 用途 |
 |------|------|------|
 | 80 | TCP | Dashboard + API + Signal + Relay |
-| 3478 | UDP | STUN (NAT 穿透) |
+| 3478 | UDP | STUN (NAT 穿透) — **本部署改用 3479；見 [stun-port-conflict.md](./stun-port-conflict.md)** |
 
 ## 常用維運指令