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/users-and-keys.md

@@ -0,0 +1,19 @@
+# Create a User and Pre-Auth Key
+
+[← 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 -c /etc/headscale/config.yaml user create homelab
+docker exec -it "$HEADSCALE_CONTAINER" headscale users list
+docker exec -it "$HEADSCALE_CONTAINER" headscale preauthkeys create --user 1 --expiration 1h
+```
+
+Copy the printed key — you'll use it once on each device you join.
+
+> **About `--expiration`:** it only limits the window in which the key can *register a new device*. Devices already joined stay authorized after the key expires. Use a longer expiration (e.g. `24h`) or `--reusable` when onboarding several devices.