docs: clarify NetBird built-in SSH vs regular SSH

Dashboard's 'SSH Access is disabled' warning confuses new users
who can clearly ssh into peers just fine. These are two different
systems: Dashboard's SSH Access is NetBird's own in-daemon SSH
tunnel server (accessed via Dashboard Connect button or
'netbird ssh'), while the regular ssh command uses the target
machine's OpenSSH running on port 22 and has nothing to do with
the Dashboard SSH toggle.

Home-lab use cases rarely need the NetBird built-in SSH — regular
OpenSSH over the NetBird overlay is sufficient. Doc explains when
you would actually want it (browser-based terminal from Dashboard,
credential offloading to NetBird identity, etc) and how to enable
if needed.

README.md

@@ -22,6 +22,7 @@
 | [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 |
+| [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 |
 
 ## 快速開始