v2.6.0: Content backup/git versioning, plugin type system, docs update

New features:
- ContentBackup class with ZIP backup/restore and git versioning
- Admin backup & restore page (content-backup.twig) with git init/commit/log/restore
- Plugin type system: system (blue) vs content (green) with visual badges
- PluginAPIInterface + AdminPluginAPI for plugin architecture
- Essential plugin flag (cannot edit/deactivate/delete)

Improvements:
- Consolidated enabled_plugins config (removed plugins.enabled)
- Removed Analytics/Logging toggles from admin config page
- Fixed Dashboard plugin Twig comments rendered as text
- Updated 20 guide files (NL+EN): configuratie, plugins, plugin-development,
  core-classes, theme-json, layouts, scss-styling, admin-beheerder, nieuw-thema, architectuur
- Improved accessibility test script (grep -E, min/max checks)

Cleanup:
- Removed unused classes: ARIAComponents, AccessibilityManager, ContentSecurityPolicy, etc.
- Removed vendor packages: mustache/mustache, php-mqtt/client
- Removed old templates: logs.twig, statistics.twig (now plugins)
- Moved language files to language/ directory

Tests:
- Pentest: 30/30 passed, 0 vulnerabilities
- WCAG 2.1 AA: 25/25 passed, 100% compliance

guide/en/admin-beheerder/configuratie.md

@@ -3,7 +3,13 @@
 ## General settings
 
 - **Site title** - Website name
-- **Default language** - nl/en/de/fr
-- **Author** - Name and email
-- **Analytics** - Visitor tracking on/off
-- **Logging** - Log files on/off
+- **Admin language** - Admin panel language (nl/en/de)
+- **Content language** - Default website content language (nl/en/de/fr)
+
+## Homepage
+
+Choose which page is shown on the homepage:
+
+- **Automatic** - First available page
+- **Most recent** - Last modified page
+- **Specific page** - Select a specific page from the content directory

guide/en/admin-beheerder/plugins.md

@@ -1,15 +1,30 @@
 # Plugins
 
+## Plugin types
+
+There are two types of plugins:
+
+- **System plugins** (blue badge) - Manage CMS functionality such as Dashboard, Logs and Statistics
+- **Content plugins** (green badge) - Display content on the front-end such as HTMLBlock and Navigation
+
+## Essential plugins
+
+Essential plugins (with a shield icon) cannot be edited, deactivated, or deleted. This applies to the Navigation plugin for example.
+
 ## Managing plugins
 
 - **Enable/Disable** - Turn plugins on/off
 - **Edit** - Modify plugin code
-- **Configuration** - Plugin settings
+- **Configuration** - Plugin settings (only for plugins with a Config button)
 - **Delete** - Remove plugin
 
+Only active plugins are shown in the admin sidebar.
+
 ## New plugin
 
 1. Go to **Plugins** → **New plugin**
 2. Enter a name (e.g. `MyPlugin`)
-3. Edit `plugin.php`
-4. Enable the plugin
+3. Edit the PHP file
+4. Enable the plugin
+
+New plugins are content-plugins by default. To create a system plugin, add `"type": "system"` to `plugin.json`.