feat: Add file names to snippets

contents/holy-grail.mdx

@@ -9,7 +9,7 @@ title: Holy grail
 
 ## HTML
 
-```html
+```html index.html
 <div class="holy-grail">
     <header>...</header>
     <main class="holy-grail__main">
@@ -28,7 +28,7 @@ title: Holy grail
 
 ## CSS
 
-```css
+```css styles.css
 .holy-grail {
     display: flex;
     flex-direction: column;