feat: add loading for markdown

Author: mortalYoung

.dumi/theme/common/Loading.tsx

@@ -0,0 +1,6 @@
+import React from 'react';
+import { Skeleton } from 'antd';
+
+export default function Loading() {
+    return <Skeleton title={false} active paragraph={{ rows: 3 }} />;
+}

loading.js

@@ -0,0 +1,3 @@
+// put it into `.dumi` folder when dumi ready
+import Loading from './.dumi/theme/common/Loading';
+export default Loading;