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

implementation/16-data-repository/src/Repository/MarkdownPageRepo.php

@@ -0,0 +1,15 @@
+<?php declare(strict_types=1);
+
+namespace Lubian\NoFramework\Repository;
+
+use Lubian\NoFramework\Exception\NotFound;
+use Lubian\NoFramework\Model\MarkdownPage;
+
+interface MarkdownPageRepo
+{
+    /** @return MarkdownPage[] */
+    public function all(): array;
+
+    /** @throws NotFound */
+    public function byName(string $name): MarkdownPage;
+}