ContentAPI voor PHP content bestanden + handleiding in admin

- Nieuwe ContentAPI class beschikbaar als $api in PHP content bestanden
  met methodes: getAllPages, getPage, getMenu, getConfig, buildUrl, etc.
- Admin handleiding pagina op /admin/guide met taalwisselaar
- Zijbalk link naar handleiding in admin menu
- Dubbele alert in config pagina verwijderd
- Handleidingen (nl/en) uitgebreid met Content API referentie

admin/templates/pages/config.php

@@ -1,12 +1,5 @@
 <h2 class="mb-4"><i class="bi bi-sliders"></i> Configuratie</h2>
 
-<?php if ($message): ?>
-    <div class="alert alert-<?= $messageType ?> alert-dismissible fade show">
-        <?= htmlspecialchars($message) ?>
-        <button type="button" class="btn-close" data-bs-dismiss="alert"></button>
-    </div>
-<?php endif; ?>
-
 <form method="POST" action="/admin/config">
     <input type="hidden" name="csrf_token" value="<?= $csrf ?>">
 

admin/templates/pages/guide.php

@@ -0,0 +1,24 @@
+<h2 class="mb-4"><i class="bi bi-book"></i> Handleiding</h2>
+
+<div class="mb-3">
+    <div class="btn-group" role="group">
+        <a href="/admin/guide?lang=nl" class="btn btn-sm <?= $lang === 'nl' ? 'btn-primary' : 'btn-outline-primary' ?>">Nederlands</a>
+        <a href="/admin/guide?lang=en" class="btn btn-sm <?= $lang === 'en' ? 'btn-primary' : 'btn-outline-primary' ?>">English</a>
+    </div>
+</div>
+
+<div class="card shadow-sm">
+    <div class="card-body guide-content">
+        <?= $content ?>
+    </div>
+</div>
+
+<style>
+.guide-content h2 { margin-top: 1.5rem; }
+.guide-content h3 { margin-top: 1.25rem; }
+.guide-content pre { background: #f4f4f4; padding: 1rem; border-radius: 4px; overflow-x: auto; }
+.guide-content code { background: #f0f0f0; padding: 0.15rem 0.4rem; border-radius: 3px; font-size: 0.9em; }
+.guide-content table { width: 100%; margin-bottom: 1rem; }
+.guide-content table th, .guide-content table td { padding: 0.5rem; border: 1px solid #dee2e6; }
+.guide-content blockquote { border-left: 3px solid #ccc; padding-left: 1rem; color: #666; margin-left: 0; }
+</style>