feat: Add file names to snippets

contents/zigzag-timeline.mdx

@@ -9,7 +9,7 @@ title: Zigzag timeline
 
 ## HTML
 
-```html
+```html index.html
 <div class="zigzag-timeline__item">
     <!-- Milestone -->
     <div className="zigzag-timeline__milestone">...</div>
@@ -24,7 +24,7 @@ title: Zigzag timeline
 
 ## CSS
 
-```css
+```css styles.css
 .zigzag-timeline__item {
     /* Used to position the milestone */
     position: relative;