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/verify-and-troubleshoot.md

@@ -0,0 +1,28 @@
+# Verify and Troubleshoot
+
+[← Back to README](../README.md)
+
+Commands below use `$HEADSCALE_CONTAINER`. Set it once per shell session on the dedicated server (change the value if your compose project name differs; find it with `docker ps`):
+
+```bash
+export HEADSCALE_CONTAINER=headscale-headscale-1
+```
+
+```bash
+docker exec -it "$HEADSCALE_CONTAINER" headscale nodes list   # on the dedicated server
+tailscale status                                             # on any client
+tailscale ping <lan-node-hostname-or-100.64.x.x-ip>          # from your laptop/phone
+```
+
+A reply of `pong via ...` (direct or DERP) means the mesh works.
+
+## `tailscale up` hangs or `/ts2021` returns 500
+
+Almost always the reverse proxy — check [NPM settings](nginx-proxy-manager.md) (Websockets on, HTTP/2 off). To confirm, watch both sides while retrying `tailscale up`:
+
+```bash
+journalctl -u tailscaled -f                 # on the client
+docker logs -f "$HEADSCALE_CONTAINER"        # on the server
+```
+
+`/ts2021` requests should return `status=200`, not `500`.