feat: add full Zonemaster stack with Docker and Spanish UI

- Clone all 5 Zonemaster component repos (LDNS, Engine, CLI, Backend, GUI)
- Dockerfile.backend: 8-stage multi-stage build LDNS→Engine→CLI→Backend
- Dockerfile.gui: Astro static build served via nginx
- docker-compose.yml: backend (internal) + frontend (port 5353)
- nginx.conf: root redirects to /es/, /api/ proxied to backend
- zonemaster-gui/config.ts: defaultLanguage set to 'es' (Spanish)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

zonemaster/docs/public/upgrading/README.md

@@ -0,0 +1,19 @@
+# Upgrading
+
+Upgrading Zonemaster is usually a straightforward process. The only component
+that may require manual intervention is [Zonemaster-Backend] and
+[Zonemaster-GUI].
+
+* See [upgrading Zonemaster-Backend] for more information when upgrading
+  [Zonemaster-Backend].
+* See [upgrading Zonemaster-GUI] for more information when upgrading
+  [Zonemaster-GUI].
+
+For other components, simply re-run the installation steps to get the updated
+version.
+
+
+[Upgrading Zonemaster-Backend]:           backend.md
+[Upgrading Zonemaster-gui]:               gui.md
+[Zonemaster-Backend]:                     https://github.com/zonemaster/zonemaster-backend
+[Zonemaster-GUI]:                         https://github.com/zonemaster/zonemaster-gui