headscale: promote from _dev, split README into per-task docs

Move step-by-step guides into headscale/docs/, slim README to a quick
reference, and use $HEADSCALE_CONTAINER for docker exec commands.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

headscale/docs/connect-clients.md

@@ -0,0 +1,36 @@
+# Connect Phone / Laptop
+
+[← Back to README](../README.md)
+
+Commands below use `$HEADSCALE_CONTAINER`. Set it once per shell session on the dedicated server (change the value if your compose project name differs; find it with `docker ps`):
+
+```bash
+export HEADSCALE_CONTAINER=headscale-headscale-1
+```
+
+## Android / iOS
+
+1. Install the Tailscale app (Play Store / App Store).
+2. Point it at your server:
+   * **Android** (tested on v1.102.3, Galaxy S25 Ultra): **Settings → Add account**, tap the **three-dot menu (⋮)**, choose the alternate/custom coordination server option, enter `https://headscale.example.com`.
+   * **iOS:** on the sign-in screen, choose **Alternate coordination server** before logging in.
+3. Log in. If the app shows a registration key instead of accepting a pre-auth key, approve it on the dedicated server:
+   ```bash
+   docker exec -it "$HEADSCALE_CONTAINER" headscale nodes register --user homelab --key <KEY-FROM-DEVICE>
+   ```
+
+## Laptop / Desktop
+
+Install the client (https://tailscale.com/download), then either use a pre-auth key:
+
+```bash
+sudo tailscale up --login-server=https://headscale.example.com --authkey <YOUR-PREAUTH-KEY>
+```
+
+Or approve interactively:
+
+```bash
+sudo tailscale up --login-server=https://headscale.example.com
+# copy the printed key, then on the dedicated server:
+docker exec -it "$HEADSCALE_CONTAINER" headscale nodes register --user homelab --key <KEY-FROM-DEVICE>
+```