Update all guides (NL + EN) for CodePress 2.5.2 features

- Admin guide: RBAC roles, role-based dashboard, plugin types (content/system)
- CodePress developer guide: plugin types, admin plugin API, SCSS compilation, asset serving
- Theme developer guide: SCSS sole CSS source, css_compiled read-only, guide layout
- Content manager guide: layout selection from theme.json, plugin order in frontmatter
- Both NL and EN updated with identical structure
- 35 files updated

guide/en/admin-beheerder/plugins.md

@@ -1,15 +1,49 @@
 # Plugins
 
+## Plugin types
+
+CodePress has two kinds of plugins, determined by the `type` field in `plugin.json`:
+
+- **Content plugins** (`type: "content"`) — Appear in the sidebar and in the plugin selection on content-edit pages. Provide sidebar content via `getSidebarContent()`.
+- **System plugins** (`type: "system"`) — Are loaded by PluginManager but do NOT appear in the sidebar. Provide functionality via admin menu, routes and the CMSAPI. Registered via `getAdminMenu()` and `getAdminRoutes()`.
+
+When no `type` field is present, `content` is assumed.
+
+## Essential plugins
+
+Essential plugins are defined in `getProtectedPlugins()` in `public/admin.php`. Current essential plugin: **Navigation**.
+
+These plugins:
+- Cannot be deactivated
+- Cannot be edited
+- Cannot be deleted
+
+On the plugins page they get an **Essential** badge instead of the action buttons.
+
 ## Managing plugins
 
-- **Enable/Disable** - Turn plugins on/off
-- **Edit** - Modify plugin code
-- **Configuration** - Plugin settings
-- **Delete** - Remove plugin
+- **Activate/Deactivate** — Turn a plugin on/off (not for essential plugins)
+- **Edit** — Modify plugin code in CodeMirror (not for essential plugins)
+- **Configuration** — Edit plugin settings
+- **Delete** — Remove the plugin folder (not for essential plugins)
 
-## New plugin
+The admin plugins page shows a **Content** or **System** badge per plugin.
+
+## Creating a new plugin
 
 1. Go to **Plugins** → **New plugin**
-2. Enter a name (e.g. `MyPlugin`)
-3. Edit `plugin.php`
-4. Enable the plugin
+2. Enter a name (e.g. `MyPlugin`) — this becomes the folder name
+3. Choose the type: **Content** or **System**
+4. The file `MyPlugin.php` is created (NOT `plugin.php`)
+5. Edit the plugin code in CodeMirror
+6. Activate the plugin
+
+## Plugin filename
+
+Plugin PHP files are named `<PluginName>.php` (e.g. `Navigation.php`, `HTMLBlock.php`). `PluginManager` loads `$pluginDir . '/' . $pluginName . '.php'`.
+
+## Plugin CSS
+
+- Content and system plugins can have their own CSS in `assets/scss/` and `assets/css/`
+- Plugin CSS is automatically loaded after theme CSS (in `base.twig`), so themes can override plugin styling
+- Plugin assets are served via `cms/router.php` at URL `/plugins/<Name>/assets/...`