add chapter about data repositories, and start work on perfomance chapter

implementation/15-adding-content/src/Model/MarkdownPage.php

@@ -0,0 +1,21 @@
+<?php declare(strict_types=1);
+
+namespace Lubian\NoFramework\Model;
+
+use Doctrine\DBAL\Types\Types;
+
+#[Entity]
+class MarkdownPage
+{
+    public function __construct(
+        #[Id,
+        Column,
+        GeneratedValue]
+        public int |null $id = null,
+        #[Column]
+        public string $title,
+        #[Column(type: Types::TEXT)]
+        public string $content,
+    ) {
+    }
+}