Add Erinnerung (reference) section with articles for Deutsch, Mathe, Englisch

- Migration, model, admin+child controllers, all views
- 17 seeded articles: Nomen, Verben, Adjektive, Zeitformen, Satzarten,
  Wortarten, Ganze Zahlen, Grundrechenarten, Einmaleins, Geometrie,
  Brüche, Zahlen/Farben/Wochentage/Sätze/Tiere auf Englisch
- CSS for .ref-content (h2/h3/p/ul/table + .beispiel/.tipp/.merke blocks)
- Child nav + admin sidebar links

app/Http/Controllers/Child/ReferenceController.php

@@ -0,0 +1,16 @@
+<?php
+namespace App\Http\Controllers\Child;
+use App\Http\Controllers\Controller;
+use App\Models\ReferenceArticle;
+class ReferenceController extends Controller {
+    public function index() {
+        $articles   = ReferenceArticle::orderBy('sort_order')->get()->groupBy('category');
+        $categories = ReferenceArticle::categories();
+        return view('child.reference.index', compact('articles','categories'));
+    }
+    public function show(ReferenceArticle $reference) {
+        $others = ReferenceArticle::where('category', $reference->category)
+            ->where('id','!=',$reference->id)->orderBy('sort_order')->get();
+        return view('child.reference.show', compact('reference','others'));
+    }
+}