docs: update documentation for Halo 2.1 (#162)

为 Halo 2.1 更新文档。

see https://github.com/halo-dev/halo/releases/tag/v2.1.0

/kind documentation

```release-note
None
```

versioned_docs/version-2.1/intro.md

@@ -0,0 +1,76 @@
+---
+id: intro
+sidebar_label: 简介
+title: ''
+sidebar_position: 1
+hide_title: true
+slug: /
+---
+
+<p align="center">
+    <a href="https://halo.run" target="_blank" rel="noopener noreferrer">
+        <img width="100" src="https://halo.run/logo" alt="Halo logo" />
+    </a>
+</p>
+
+<p align="center"><b>Halo</b> [ˈheɪloʊ]，好用又强大的开源建站工具。</p>
+
+<p align="center">
+<a href="https://github.com/halo-dev/halo/releases"><img alt="GitHub release" src="https://img.shields.io/github/release/halo-dev/halo.svg?style=flat-square&include_prereleases" /></a>
+<a href="https://github.com/halo-dev/halo/releases"><img alt="GitHub All Releases" src="https://img.shields.io/github/downloads/halo-dev/halo/total.svg?style=flat-square" /></a>
+<a href="https://hub.docker.com/r/halohub/halo"><img alt="Docker pulls" src="https://img.shields.io/docker/pulls/halohub/halo?style=flat-square" /></a>
+<a href="https://github.com/halo-dev/halo/commits"><img alt="GitHub last commit" src="https://img.shields.io/github/last-commit/halo-dev/halo.svg?style=flat-square" /></a>
+<a href="https://github.com/halo-dev/halo/actions"><img alt="GitHub Workflow Status" src="https://img.shields.io/github/actions/workflow/status/halo-dev/halo/halo.yaml?branch=main&style=flat-square" /></a>
+<br />
+<a href="https://halo.run">官网</a>
+<a href="https://docs.halo.run">文档</a>
+<a href="https://bbs.halo.run">社区</a>
+<a href="https://gitee.com/halo-dev">Gitee</a>
+<a href="https://t.me/halo_dev">Telegram 频道</a>
+</p>
+
+------------------------------
+
+## 快速开始
+
+```bash
+docker run \
+  -it -d \
+  --name halo \
+  -p 8090:8090 \
+  -v ~/.halo2:/root/.halo2 \
+  -e HALO_EXTERNAL_URL=http://localhost:8090/ \
+  -e HALO_SECURITY_INITIALIZER_SUPERADMINUSERNAME=admin \
+  -e HALO_SECURITY_INITIALIZER_SUPERADMINPASSWORD=P@88w0rd \
+  halohub/halo:2.0
+```
+
+以上仅作为体验使用，详细部署文档请查阅：<https://docs.halo.run/getting-started/install/docker-compose>
+
+## 在线体验
+
+- 环境地址：<https://demo.halo.run>
+- 后台地址：<https://demo.halo.run/console>
+- 用户名：`demo`
+- 密码：`P@ssw0rd123..`
+
+## 生态
+
+可访问 [awesome-halo](https://github.com/halo-sigs/awesome-halo) 查看已经适用于 Halo 2.0 的主题和插件，以及适用于 Halo
+1.x 的相关仓库。
+
+## 许可证
+
+[![license](https://img.shields.io/github/license/halo-dev/halo.svg?style=flat-square)](https://github.com/halo-dev/halo/blob/master/LICENSE)
+
+Halo 使用 GPL-v3.0 协议开源，请遵守开源协议。
+
+## 贡献
+
+参考 [CONTRIBUTING](https://github.com/halo-dev/halo/blob/master/CONTRIBUTING.md)。
+
+<a href="https://github.com/halo-dev/halo/graphs/contributors"><img src="https://opencollective.com/halo/contributors.svg?width=890&button=false" /></a>
+
+## 状态
+
+![Repobeats analytics](https://repobeats.axiom.co/api/embed/ad008b2151c22e7cf734d2688befaa795d593b95.svg "Repobeats analytics image")