feat: Add file names to snippets

contents/full-background.mdx

@@ -9,13 +9,13 @@ title: Full background
 
 ## HTML
 
-```html
+```html index.html
 <div class="full-background">...</div>
 ```
 
 ## CSS
 
-```css
+```css styles.css
 .full-background {
     /* Center the content */
     align-items: center;