website: add backToUp button.

Author: jaywcjlove

package.json

@@ -75,6 +75,7 @@
     "@types/react": "~18.0.9",
     "@types/react-dom": "~18.0.3",
     "@types/react-test-renderer": "~18.0.0",
+    "@uiw/react-back-to-top": "^1.2.0",
     "@uiw/react-codesandbox": "~1.1.4",
     "@uiw/react-github-corners": "~1.5.3",
     "@uiw/react-shields": "~1.1.2",

website/App.tsx

@@ -2,6 +2,7 @@ import { useState } from 'react';
 import GitHubCorners from '@uiw/react-github-corners';
 import Github from '@uiw/react-shields/esm/github';
 import Npm from '@uiw/react-shields/esm/npm';
+import BackToUp from '@uiw/react-back-to-top';
 import '@wcj/dark-mode';
 import logo from './logo.svg';
 import './App.css';
@@ -14,6 +15,7 @@ const App = () => {
   const [value, setValue] = useState('');
   return (
     <div className="App">
+      <BackToUp>Top</BackToUp>
       <dark-mode permanent light="Light" dark="Dark"></dark-mode>
       <GitHubCorners zIndex={9999} fixed target="__blank" href="https://github.com/uiwjs/react-markdown-preview" />
       <header className="App-header">