feat: Add file names to snippets

contents/sticky-sections.mdx

@@ -9,7 +9,7 @@ title: Sticky sections
 
 ## HTML
 
-```html
+```html index.html
 <div class="sticky-sections">
     <section class="sticky-sections__section">...</section>
 
@@ -20,7 +20,7 @@ title: Sticky sections
 
 ## CSS
 
-```css
+```css styles.css
 .sticky-sections {
     height: 100%;
     overflow: scroll;