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/README.md

@@ -0,0 +1,59 @@
+# Public documentation
+
+*Click on [SUMMARY](SUMMARY.md) to view a linked tree structure of the public
+documentation.*
+
+<!-- A copy of the text below is found in ../README-for-doc.zonemaster.net.md
+and if any update is done here, make sure to update there too. -->
+
+The public documentation is divided into the following main categories:
+
+* [Release information](RELEASE.md) gives release information of latest release.
+* [Installation](installation/README.md) contains documents that explain how to
+  install the different components of Zonemaster.
+* [Upgrading](upgrading/README.md) contains documents that explain how to upgrade
+  a Zonemaster installation.
+* [Configuration](configuration/README.md) contains documents that explain how a
+  Zonemaster installation can be configured.
+* [Using](using/README.md) contains user guides to the different components of
+  Zonemaster.
+* [Specifications](specifications/README.md) contains specifications of the
+  Zonemaster test cases, test types and test zones.
+* [Development](development/README.md) contains information for developers both
+  in and outside the Zonemaster project.
+* [License](LICENSE.md) gives the license information for Zonemaster.
+
+## Rendering
+
+Renderings of the public documentation are published to [doc.zonemaster.net] for
+every release version since v2023.1. It can be built locally using [mdbook], its
+[mdbook-linkcheck] plugin.
+
+1. Get a copy of the source tree by cloning the [repository] or downloading an
+archive of one of the [releases].
+2. Change to the `docs/public` directory and build the book.
+```
+cd docs/public
+mdbook build
+```
+3. To view the content there are a few options:
+
+* Open the index file if your OS has support for it:
+```
+open book/index.html
+```
+* Let mdbook serve it on local computer. Run the following command and open
+  [localhost:3000] in your browser.
+```
+mdbook serve
+```
+* Copy the `book` directory with all files to your computer, go directly to
+  that directory and open `book/index.html`.
+
+
+[doc.zonemaster.net]: https://doc.zonemaster.net
+[localhost:3000]: http://localhost:3000
+[releases]: https://github.com/zonemaster/zonemaster/releases
+[repository]: https://github.com/zonemaster/zonemaster
+[mdbook]:                              https://rust-lang.github.io/mdBook/
+[mdbook-linkcheck]:                    https://github.com/Michael-F-Bryan/mdbook-linkcheck