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/config.yaml

@@ -0,0 +1,33 @@
+server_url: https://headscale.example.com
+listen_addr: 0.0.0.0:8080
+
+noise:
+  private_key_path: /var/lib/headscale/noise_private.key
+
+database:
+  type: sqlite
+  sqlite:
+    path: /var/lib/headscale/db.sqlite
+
+prefixes:
+  v4: 100.64.0.0/10
+  v6: fd7a:115c:a1e0::/48
+
+derp:
+  server:
+    enabled: false
+  urls:
+    - https://controlplane.tailscale.com/derpmap/default
+  auto_update_enabled: true
+  update_frequency: 24h
+
+log:
+  level: info
+
+dns:
+  override_local_dns: true
+  base_domain: tailnet.local
+  nameservers:
+    global:
+      - 1.1.1.1
+      - 8.8.8.8