文档: 新增 Docusaurus 官网与双语文档，README 切换为英文默认 (#39)

- 新建 docs-site/ Docusaurus 项目，支持 en + zh-Hans 双语
- 从 README 迁移内容为独立文档页面：
  - Getting Started（安装、快速开始）
  - Deployment（Docker、裸机、Nginx、配置参考）
  - Features（备份类型、存储后端、SAP HANA、多节点集群、通知）
  - Reference（API、CLI）
  - Development（开发、贡献）
- 自定义 BackupX 主题色、logo、落地页组件
- 新增 .github/workflows/docs.yml，Actions 自动构建并发布到 GitHub Pages
- README.md 切换为英文，中文版挪到 README.zh-CN.md，两者均精简为导航型
- 配置站点 URL：https://awuqing.github.io/BackupX/

docs-site/README.md

@@ -0,0 +1,41 @@
+# Website
+
+This website is built using [Docusaurus](https://docusaurus.io/), a modern static website generator.
+
+## Installation
+
+```bash
+yarn
+```
+
+## Local Development
+
+```bash
+yarn start
+```
+
+This command starts a local development server and opens up a browser window. Most changes are reflected live without having to restart the server.
+
+## Build
+
+```bash
+yarn build
+```
+
+This command generates static content into the `build` directory and can be served using any static contents hosting service.
+
+## Deployment
+
+Using SSH:
+
+```bash
+USE_SSH=true yarn deploy
+```
+
+Not using SSH:
+
+```bash
+GIT_USER=<Your GitHub username> yarn deploy
+```
+
+If you are using GitHub pages for hosting, this command is a convenient way to build the website and push to the `gh-pages` branch.