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/installation/README.md

@@ -0,0 +1,32 @@
+# Installation instructions
+
+## Manual installation
+
+Before installing Zonemaster, check the [prerequisites] first.
+
+Start by installing the following components, in order:
+
+ * [Zonemaster-LDNS];
+ * [Zonemaster-Engine];
+ * [Zonemaster-CLI].
+
+At this point, tests can be run from the command-line on the local machine.
+To install the Web GUI, install the following additional components, in order:
+
+ * [Zonemaster-Backend];
+ * [Zonemaster-GUI].
+
+## Docker
+
+Most Zonemaster components are available as [Docker] containers. See the dedicated
+[Docker document] for more information.
+
+[Docker]:                              https://www.docker.com/get-started/
+[Docker document]:                     docker.md
+[prerequisites]:                       prerequisites.md
+[prerequisites]:                       prerequisites.md
+[Zonemaster-Backend]:                  zonemaster-backend.md
+[Zonemaster-CLI]:                      zonemaster-cli.md
+[Zonemaster-Engine]:                   zonemaster-engine.md
+[Zonemaster-GUI]:                      zonemaster-gui.md
+[Zonemaster-LDNS]:                     zonemaster-ldns.md