docs: update types for theme config (#3118)

* docs: update types for theme config

docs/guide/extending-default-theme.md

@@ -115,12 +115,10 @@ import type { Theme } from 'vitepress'
 import DefaultTheme from 'vitepress/theme'
 
 export default {
-  // ...
+  extends: DefaultTheme,
   async enhanceApp({ app }) {
-    if (!import.meta.env.SSR) {
-      const plugin = await import('plugin-that-access-window-on-import')
-      app.use(plugin)
-    }
+    // register your custom global components
+    ctx.app.component('MyGlobalComponent' /* ... */)
   }
 } satisfies Theme
 ```