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/install.md

@@ -0,0 +1,24 @@
+# Install Headscale
+
+[← Back to README](../README.md)
+
+Run on the **dedicated server** (public IP).
+
+```bash
+# Copy the starter config into place, then edit it
+mkdir -p /data/headscale/config
+cp config.yaml /data/headscale/config/config.yaml
+nano /data/headscale/config/config.yaml   # set server_url to YOUR domain, e.g. https://headscale.example.com
+
+# Copy env defaults and start the stack
+cp sample.env .env
+docker compose up -d
+```
+
+Headscale will not start without a valid `config.yaml`, and `server_url` **must** match your public domain.
+
+Next: [configure Nginx Proxy Manager](nginx-proxy-manager.md), then verify:
+
+```bash
+curl https://headscale.example.com/health
+```