docs: update quick start instructions for yarn v1 (#2310)

Author: jay-es

src/guide/quick-start.md

@@ -46,6 +46,9 @@ import { VTCodeGroup, VTCodeGroupTab } from '@vue/theme'
   <VTCodeGroupTab label="yarn">
 
   ```sh
+  # Yarn (v1+)
+  $ yarn create vue
+
   # Yarn (v2+)
   $ yarn create vue@latest