v2.5.1: Admin theme refactor, Navigation plugin, user roles, guide restructure

- Reorganize admin into admin/theme/default/ (views + assets)
- Rename GuideNav to Navigation plugin (essential, protected)
- Plugin assets support (SCSS/CSS) loaded after theme CSS
- User roles: Admin, Content Manager, BI Manager, Site Admin
- Role-based access control (RBAC) for admin routes and sidebar
- Guide restructure: sub-topics in separate folders with sidebar nav
- Dynamic breadcrumb for homepage and subdirectories
- Fix theme path traversal (../../ -> ../) in admin.php
- Fix CodeMirror mode load order (xml -> css -> js -> htmlmixed -> php)
- Fix editor-toolbar.js null checks for plugin edit pages
- Layout select from theme.json with live frontmatter update
- Footer sticky at bottom of viewport (min-height: 100vh)
- Breadcrumb color fix (var(--nav-font) -> var(--header-bg))
- Remove language switcher from guide pages
- Update README.md and README.en.md
- Bump version to 2.5.1

guide/nl/codepress-developer/plugin-development.md

@@ -0,0 +1,43 @@
+# Plugin Development
+
+## Plugin structuur
+
+```
+plugins/MijnPlugin/
+├── plugin.json         # Plugin metadata
+├── plugin.php          # Plugin code
+└── config.json         # Optionele configuratie
+```
+
+## plugin.json
+
+```json
+{
+  "name": "Mijn Plugin",
+  "version": "1.0.0",
+  "author": "Jouw Naam",
+  "description": "Beschrijving"
+}
+```
+
+## plugin.php voorbeeld
+
+```php
+<?php
+/**
+ * Plugin: MijnPlugin
+ */
+
+echo '<div class="mijn-plugin">Hello World</div>';
+```
+
+## CMSAPI gebruiken
+
+```php
+<?php
+require_once '../../cms/core/class/PluginManager.php';
+
+$api = PluginManager::getAPI();
+$config = $api->getConfig();
+$content = $api->getContent();
+```