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/src/pages/index.module.css

@@ -0,0 +1,31 @@
+.heroBanner {
+  padding: 5rem 0 4rem;
+  text-align: center;
+  position: relative;
+  overflow: hidden;
+}
+
+@media screen and (max-width: 996px) {
+  .heroBanner {
+    padding: 3rem 1rem;
+  }
+}
+
+.buttons {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  gap: 1rem;
+  flex-wrap: wrap;
+  margin-top: 1.5rem;
+}
+
+.secondaryBtn {
+  color: #fff !important;
+  border-color: #fff;
+}
+
+.secondaryBtn:hover {
+  background-color: rgba(255, 255, 255, 0.15);
+  color: #fff;
+}

docs-site/src/pages/index.tsx

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+import type {ReactNode} from 'react';
+import clsx from 'clsx';
+import Link from '@docusaurus/Link';
+import Translate, {translate} from '@docusaurus/Translate';
+import useDocusaurusContext from '@docusaurus/useDocusaurusContext';
+import Layout from '@theme/Layout';
+import Heading from '@theme/Heading';
+import HomepageFeatures from '@site/src/components/HomepageFeatures';
+
+import styles from './index.module.css';
+
+function HomepageHeader() {
+  return (
+    <header className={clsx('hero hero--primary', styles.heroBanner)}>
+      <div className="container">
+        <Heading as="h1" className="hero__title">
+          BackupX
+        </Heading>
+        <p className="hero__subtitle">
+          <Translate id="home.tagline">
+            Self-hosted server backup management — one binary, one command, manage every backup
+          </Translate>
+        </p>
+        <div className={styles.buttons}>
+          <Link
+            className="button button--secondary button--lg"
+            to="/docs/getting-started/quick-start">
+            <Translate id="home.getStarted">Get Started</Translate>
+          </Link>
+          <Link
+            className={clsx('button button--outline button--secondary button--lg', styles.secondaryBtn)}
+            to="https://github.com/Awuqing/BackupX">
+            GitHub
+          </Link>
+        </div>
+      </div>
+    </header>
+  );
+}
+
+export default function Home(): ReactNode {
+  const {siteConfig} = useDocusaurusContext();
+  return (
+    <Layout
+      title={translate({id: 'home.title', message: 'Self-hosted backup management'})}
+      description={siteConfig.tagline}>
+      <HomepageHeader />
+      <main>
+        <HomepageFeatures />
+      </main>
+    </Layout>
+  );
+}