docs: add NetBird exit nodes via CT100 (TW) and virmach-lax (US)

Covers 0.0.0.0/0 route creation, opt-in client-side selection (different
from subnet routes which auto-apply), and the extra trust exposure when
routing through a third-party VPS.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -21,7 +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 |
+| [network-routes.md](./network-routes.md) | Subnet Router（`home-lan`）與 Exit Nodes（`exit-ct100`、`exit-virmach-lax`） |
 | [ssh-access.md](./ssh-access.md) | 一般 SSH vs NetBird 內建 SSH — 常見混淆點釐清 |
 | [SUMMARY.md](./SUMMARY.md) | 部署摘要與疑難排解記錄 |