Add Kiwix container installer and metadata

Introduce a complete Kiwix LXC installer: ct/kiwix.sh (container template) and install/kiwix-install.sh (in-container install). The installer installs kiwix-tools from the Kiwix PPA, creates a systemd kiwix-serve service to serve /data/*.zim on port 8080, records the package version in /root/.kiwix, and includes an update_script to upgrade kiwix-tools. The container template validates a provided ZIM_DATA directory, configures a bind mount (using ID-mapped mounts if available), sets container options, and prints the service URL. Also add json/kiwix.json catalog metadata with usage notes (including ZIM_DATA usage and file permission guidance).

json/kiwix.json

@@ -0,0 +1,44 @@
+{
+    "name": "Kiwix",
+    "slug": "kiwix",
+    "categories": [
+        0
+    ],
+    "date_created": "2026-02-11",
+    "type": "ct",
+    "updateable": true,
+    "privileged": false,
+    "interface_port": 8080,
+    "documentation": "https://wiki.kiwix.org",
+    "website": "https://www.kiwix.org",
+    "logo": "https://cdn.jsdelivr.net/gh/selfhst/icons@main/webp/kiwix.webp",
+    "description": "Offline content browser serving Wikipedia, Project Gutenberg, and more from ZIM archives",
+    "config_path": "",
+    "install_methods": [
+        {
+            "type": "default",
+            "script": "ct/kiwix.sh",
+            "resources": {
+                "cpu": 1,
+                "ram": 512,
+                "hdd": 4,
+                "os": "Ubuntu",
+                "version": "24.04"
+            }
+        }
+    ],
+    "default_credentials": {
+        "username": null,
+        "password": null
+    },
+    "notes": [
+        {
+            "text": "Requires a bind-mounted directory with ZIM archives. Download some from https://library.kiwix.org to /your/zims. The installation command is then: `ZIM_DATA=/your/zims bash -c \"$(curl .../ct/kiwix.sh)\"`.",
+            "type": "info"
+        },
+        {
+            "text": "Ensure ZIM files are world-readable: chmod -R a+rX /your/zims",
+            "type": "warning"
+        }
+    ]
+}