docs(i18n): 恢复并补齐英文版 vitepress，README 默认改为英文

- README 默认英文：README.en.md → README.md（英文为默认），中文移至 README.zh-CN.md，语言切换链接同步
- 恢复被删除的 docs/en/ 英文文档（git 历史 cc5e53c5^），删除 4 篇已废弃文件
- 英文导航 config.ts 对齐中文结构（新增 Deployment/Changelog 侧栏，同步 Guide/Design 条目）
- 翻译 15 篇中文新增文档：guide 5 篇（certificates/pages-usage/proxy-config/uptime-kuma/zone-domain-migration）+ design 10 篇（zone-design/cloudflare-pointing/waf-orchestration/origin-error-page/edge-cache-design/pages-design/logstore/kuma-design/login-captcha/observability 三篇）
- en 首页更新（新增 Pages 特性、tagline 同步）；changelog 英文入口指向中文版
- vitepress 构建验证：43 个英文页面全部渲染

注意：29 篇旧英文文档为恢复版，部分内容（如 deployment/server、reference/configuration）可能落后于中文，需后续逐篇同步

docs/en/reference/index.md

@@ -0,0 +1,13 @@
+# Reference Manuals
+
+You will learn: Which information belongs to stable reference manuals, and where to look up configurations, commands, APIs, and repository structures.
+
+This section collects stable information at the runtime, API, and repository layers, suitable for rapid lookup during deployment, integration, and troubleshooting.
+
+| Page | Content |
+| --- | --- |
+| [Configuration Options](./configuration.md) | Server environment variables, CLI arguments, runtime Options, and Agent configuration parameters |
+| [CLI Commands](./cli.md) | Common CLI commands for starting, building, testing, installing, and uninstalling |
+| [API Conventions](./api.md) | Response structures, authentication, and routing paths for Admin and Agent APIs |
+| [Repository Structure](../design/repository.md) | Scope of responsibilities and folder layering of the Server, Agent, Relay, and Client |
+| [Deployment & Upgrade](../deployment/) | Server and Agent deployment, configuration, and upgrade guides (dedicated section) |