docs: organize contents into "Introduction" and "Advanced"

docs/.vitepress/config.js

@@ -37,10 +37,15 @@ function getGuideSidebar() {
         { text: 'Getting Started', link: '/guide/getting-started' },
         { text: 'Configuration', link: '/guide/configuration' },
         { text: 'Markdown Extensions', link: '/guide/markdown' },
-        { text: 'Frontmatter', link: '/guide/frontmatter' },
-        { text: 'Customization', link: '/guide/customization' },
         { text: 'Deploying', link: '/guide/deploy' }
       ]
+    },
+    {
+      text: 'Advanced',
+      children: [
+        { text: 'Frontmatter', link: '/guide/frontmatter' },
+        { text: 'Customization', link: '/guide/customization' }
+      ]
     }
   ]
 }