add 'adding content' chapter

app/src/Template/GithubMarkdownRenderer.php

@@ -0,0 +1,28 @@
+<?php declare(strict_types=1);
+
+namespace Lubian\NoFramework\Template;
+
+use League\CommonMark\Environment\Environment;
+use League\CommonMark\Extension\CommonMark\CommonMarkCoreExtension;
+use League\CommonMark\Extension\GithubFlavoredMarkdownExtension;
+use League\CommonMark\MarkdownConverter;
+
+final class GithubMarkdownRenderer implements MarkdownRenderer
+{
+    private MarkdownConverter $engine;
+
+    public function __construct(
+        CommonMarkCoreExtension $commonMarkCoreExtension,
+        GithubFlavoredMarkdownExtension $githubFlavoredMarkdownExtension,
+    ) {
+        $environment = new Environment([]);
+        $environment->addExtension($commonMarkCoreExtension);
+        $environment->addExtension($githubFlavoredMarkdownExtension);
+        $this->engine = new MarkdownConverter($environment);
+    }
+
+    public function render(string $markdown): string
+    {
+        return (string) $this->engine->convert($markdown);
+    }
+}