docs: add information about setup in theme

docs/guide/theme-introduction.md

@@ -38,6 +38,7 @@ interface Theme {
   Layout: Component // Vue 3 component
   NotFound?: Component
   enhanceApp?: (ctx: EnhanceAppContext) => void
+  setup?: () => void
 }
 
 interface EnhanceAppContext {
@@ -65,6 +66,11 @@ export default {
     // router is VitePress' custom router. `siteData` is
     // a `ref` of current site-level metadata.
   }
+
+  setup() {
+    // this function will be executed inside VitePressApp's
+    // setup hook. all composition APIs are available here.
+  }
 }
 ```