feat: Add file names to snippets

contents/tree-diagram.mdx

@@ -9,7 +9,7 @@ title: Tree diagram
 
 ## HTML
 
-```html
+```html index.html
 <div class="tree-diagram">
     <ul>
         <li>
@@ -42,7 +42,7 @@ title: Tree diagram
 
 ## CSS
 
-```css
+```css styles.css
 .tree-diagram ul {
     display: flex;
     position: relative;