docs: add vercel analytics

docs/src/.vitepress/theme/index.ts

@@ -1,6 +1,7 @@
 import type { Theme } from 'vitepress'
 import VueAnimXyz from '@animxyz/vue3'
 import DefaultTheme from 'vitepress/theme'
+import { webVitals } from '../plugins/vercel-web-vitals-api'
 import Layout from './layouts/default.vue'
 import 'virtual:windi.css'
 import '@animxyz/core'
@@ -13,6 +14,7 @@ const CustomTheme = {
   Layout,
   enhanceApp: ({ app }) => {
     app.use(VueAnimXyz)
+    webVitals({ analyticsId: '__ANALYTICS_ID__', debug: true })
   },
 } as Theme