docs: update readme for Halo 2.0 (#2492)

#### What type of PR is this?

/kind documentation
/milestone 2.0

#### What this PR does / why we need it:

更新 README。

See https://github.com/halo-dev/halo/issues/2410 for more

#### Special notes for your reviewer:

/hold

#### Does this PR introduce a user-facing change?

```release-note
None
```

README.md

@@ -7,14 +7,14 @@
 <p align="center"><b>Halo</b> [ˈheɪloʊ]，一款现代化的开源博客/CMS系统，值得一试。</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" /></a>
+<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/workflow/status/halo-dev/halo/Halo%20CI?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://docs.halo.run/2.0.0-SNAPSHOT">文档（2.0 Alpha）</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>
@@ -22,6 +22,14 @@
 
 ------------------------------
 
+## 注意
+
+当前分支为 Halo 2.0 的开发分支，目前 Halo 2.0 处于 Alpha 测试阶段，无法从 1.5 直接升级，也不建议在生产环境使用。稳定版本（Halo 1.x）请查阅以下地址：
+
+- 1.5 分支：<https://github.com/halo-dev/halo/tree/release-1.5>
+- 1.6 分支：<https://github.com/halo-dev/halo/tree/release-1.6>
+- 1.5 文档：<https://docs.halo.run>
+
 ## 快速开始
 
 ### Fat Jar
@@ -29,42 +37,26 @@
 下载最新的 Halo 运行包：
 
 ```bash
-curl -L https://github.com/halo-dev/halo/releases/download/v1.4.17/halo-1.4.17.jar --output halo.jar
+curl -L https://github.com/halo-dev/halo/releases/download/v2.0.0-alpha.1/halo-2.0.0-alpha.1.jar --output halo-next.jar
 ```
 
-其他地址：<https://docs.halo.run/getting-started/downloads>
+其他地址：<https://docs.halo.run/next/getting-started/downloads>
 
 ```bash
-java -jar halo.jar
+java -jar halo-next.jar
 ```
 
 ### Docker
 
 ```bash
-docker run -it -d --name halo -p 8090:8090 -v ~/.halo:/root/.halo --restart=always halohub/halo
+docker run -it -d --name halo-next -p 8090:8090 -v ~/halo-next:/root/halo-next --restart=unless-stopped halohub/halo-dev:next
 ```
 
-详细部署文档请查阅：<https://docs.halo.run/getting-started/install/linux>
-
-## 在线体验
-
-- 环境地址：<https://demo.halo.run>
-- 后台地址：<https://demo.halo.run/admin>
-- 用户名：demo
-- 密码：P@ssw0rd123..
-- 使用前请阅读：<https://demo.halo.run/archives/tips>
+详细部署文档请查阅：<https://docs.halo.run/next/getting-started/install/linux>
 
 ## 生态
 
-| 项目                                                                         | 状态                                                                                                                                                              | 描述                                     |
-| ---------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------------------------------------- |
-| [halo-admin](https://github.com/halo-dev/halo-admin)                         | <a href="https://www.npmjs.com/package/halo-admin"><img alt="npm release" src="https://img.shields.io/npm/v/halo-admin?style=flat-square"/></a>                   | Web 管理端 UI，已内置在主应用            |
-| [js-sdk](https://github.com/halo-dev/js-sdk)                                 | <a href="https://github.com/halo-dev/js-sdk"><img alt="npm release" src="https://img.shields.io/npm/v/@halo-dev/content-api?style=flat-square"/></a>                           | JavaScript SDK                           |
-| [halo-comment](https://github.com/halo-dev/halo-comment)                     | <a href="https://www.npmjs.com/package/halo-comment"><img alt="npm release" src="https://img.shields.io/npm/v/halo-comment?style=flat-square"/></a>               | 独立评论组件，可以非常方便的集成到主题中 |
-| [halo-comment-normal](https://github.com/halo-dev/halo-comment-normal)       | <a href="https://www.npmjs.com/package/halo-comment-normal"><img alt="npm release" src="https://img.shields.io/npm/v/halo-comment-normal?style=flat-square"/></a> | 另外一款评论组件                         |
-| [halo-mobile-app](https://github.com/halo-dev/halo-mobile-app)               | 已停止维护                                                                                                                                                        | 移动端管理 APP                           |
-| [tencent-cloudbase-halo](https://github.com/halo-dev/tencent-cloudbase-halo) | 无                                                                                                                                                                | 腾讯云 CloudBase 一键部署配置            |
-| [halo-theme-\*](https://github.com/topics/halo-theme)                        | 无                                                                                                                                                                | GitHub 上开源的 Halo 主题集合            |
+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