docs: add sidebar docu with examples

docs/guide/configuration.md

@@ -1,5 +1,7 @@
 # Configuration
 
+## Basics
+
 Without any configuration, the page is pretty minimal, and the user has no way to navigate around the site. To customize your site, let’s first create a `.vitepress` directory inside your docs directory. This is where all VitePress-specific files will be placed. Your project structure is probably like this:
 
 ```bash
@@ -20,4 +22,69 @@ module.exports = {
 }
 ```
 
+## Sidebar
+
+The sidebar is empty by default. You can configure it as part of the theme in the `.vitepress/config` like this:
+
+```js
+module.exports = {
+  title: 'Hello VitePress',
+  // other config here...
+
+  themeConfig: {
+    sidebar: {
+      '/': [
+        {
+          text: 'Introduction',
+          children: [
+            { text: 'What is VitePress?', link: '/' },
+            { text: 'Getting Started', link: '/guide/getting-started' },
+            // more items...
+          ]
+        },
+        {
+          text: 'Advanced',
+          children: [
+            { text: 'Frontmatter', link: '/guide/frontmatter' },
+            // more items...
+          ]
+        }
+      ]
+    }
+  }
+}
+```
+
+Notice that this configuration only applies for the route `/`. When the user navigates to `/guide` the sidebar would appear empty. Each route can/must define their own `SidebarItem`.
+
+If you want to display the same sidebar content for different routes, consider creating a function. To share the sidebar content for route `/` and`/guide` see the following example:
+
+```js
+// ..
+  themeConfig: {
+    sidebar: {
+      '/guide/': getGuideSidebar(),
+      '/': getGuideSidebar()
+    }
+  }
+}
+
+function getGuideSidebar() {
+  return [
+    {
+      text: 'Introduction',
+      children: [
+        { text: 'What is VitePress?', link: '/' },
+        // more items...
+      ]
+    },
+    // like above
+  ]
+}
+```
+
+Even so vitepress does not support all features of Vuepress, you might find the [Vuepress documentation about the sidebar](https://vuepress.vuejs.org/theme/default-theme-config.html#sidebar) helpfull.
+
+## Config Reference
+
 Check out the [Config Reference](/config/basics) for a full list of options.