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

cms/core/config.php

@@ -24,10 +24,11 @@ if (!file_exists($configJsonPath)) {
             ],
             'author' => [
                 'name' => 'E. Noorlander',
-                'website' => 'noorlander.info'
+                'website' => 'noorlander.info',
+                'git' => ''
             ],
             'show_version' => true,
-            'enabled_plugins' => ['MQTTTracker', 'HTMLBlock'],
+            'enabled_plugins' => ['HTMLBlock', 'Navigation'],
             'features' => [
                 'auto_link_pages' => true,
                 'search_enabled' => true,