docs: always use 'extends' to extend themes (#3181)

docs/guide/extending-default-theme.md

@@ -135,7 +135,7 @@ import DefaultTheme from 'vitepress/theme'
 import MyLayout from './MyLayout.vue'
 
 export default {
-  ...DefaultTheme,
+  extends: DefaultTheme,
   // override the Layout with a wrapper component that
   // injects the slots
   Layout: MyLayout
@@ -168,7 +168,7 @@ import DefaultTheme from 'vitepress/theme'
 import MyComponent from './MyComponent.vue'
 
 export default {
-  ...DefaultTheme,
+  extends: DefaultTheme,
   Layout() {
     return h(DefaultTheme.Layout, null, {
       'aside-outline-before': () => h(MyComponent)

docs/guide/i18n.md

@@ -82,7 +82,7 @@ However, VitePress won't redirect `/` to `/en/` by default. You'll need to confi
 import DefaultTheme from 'vitepress/theme'
 
 export default {
-  ...DefaultTheme,
+  extends: DefaultTheme,
   setup() {
     const { lang } = useData()
     watchEffect(() => {