chore: add alert plugin for docs; 🌟

docs/.vuepress/plugins/alert/index.js

@@ -0,0 +1,13 @@
+const path = require('path')
+
+module.exports = (options, ctx) => {
+  return {
+    clientRootMixin: path.resolve(__dirname, 'clientRootMixin.js'),
+    extendPageData($page) {
+      $page.alert = {
+        top: 100
+      }
+    },
+    enhanceAppFiles: path.resolve(__dirname, 'enhanceApp.js')
+  }
+}