Optimize the 404 page (#216)

Fix 404 party of #213

<img width="1386" alt="2743f73507eb76ce0b749e9d804e34c.png" src="attachments/7dbeeab8-6e31-405e-8dc8-917e4a843657">

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-on: https://gitea.com/gitea/docs/pulls/216
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Kerwin Bryant <kerwin612@qq.com>
Co-committed-by: Kerwin Bryant <kerwin612@qq.com>

src/theme/NotFound/Content/index.js

@@ -0,0 +1,16 @@
+import React from 'react';
+import Content from '@theme-original/NotFound/Content';
+import styles from './styles.module.css';
+
+export default function ContentWrapper(props) {
+  return (
+    <>
+      <Content {...props} />
+      <div className={styles.links}>
+        <a href="/">Browse the latest docs</a>
+        <span className={styles.separator}/>
+        <a href="/next">Access next docs</a>
+      </div>
+    </>
+  );
+}

src/theme/NotFound/Content/styles.module.css

@@ -0,0 +1,15 @@
+.links {
+  width: 100%;
+  margin-top: -80px;
+  display: flex;
+  gap: 20px;
+  align-items: center;
+  justify-content: center;
+}
+
+.separator{
+  width: 5px;
+  height: 5px;
+  border-radius: 50%;
+  background-color: black;
+}