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/using/README.md

@@ -0,0 +1,30 @@
+# Zonemaster users guide
+
+The purpose of the Zonemaster project is to develop a DNS validation software.
+There are three ways one can use the Zonemaster software:
+
+   * Using the Web user Interface: In this case, there is no installation of
+     the Zonemaster software needed. A user types the URL
+     [https://zonemaster.net][zonemaster.net], provides as input a domain name,
+     and then can view the results.
+   * From the Command Line Interface (CLI): In this case, the user has to
+     install two components of the Zonemaster software, i.e. the
+     [Zonemaster-Engine] and the [Zonemaster-CLI].
+   * There are cases, wherein an organisation or a user, that wants to have the
+     results of their DNS validation stored and can be viewed later. In that
+     case, in addition to the Zonemaster-Engine and Zonemaster-CLI, the user
+     has to install the [Zonemaster-Backend] and the [Zonemaster-GUI]. 
+
+User guides:
+ * [Use Zonemaster-CLI]
+ * [Use Zonemaster Backend]
+ * [Use Zonemaster-GUI]
+
+[Use Zonemaster Backend]:    backend/README.md
+[Use Zonemaster-CLI]:        cli.md
+[Use Zonemaster-GUI]:        gui/README.md
+[Zonemaster-Backend]:        ../installation/zonemaster-backend.md
+[Zonemaster-CLI]:            ../installation/zonemaster-cli.md
+[Zonemaster-Engine]:         ../installation/zonemaster-engine.md
+[Zonemaster-GUI]:            ../installation/zonemaster-gui.md
+[zonemaster.net]:            https://zonemaster.net/