docs: restructure Get Started tab and improve onboarding flow (#9950)

* docs: restructure Get Started tab and improve onboarding flow

- Flatten nested Onboarding group into linear First Steps flow
- Add 'What is OpenClaw?' narrative section to landing page
- Split wizard.md into streamlined overview + full reference (reference/wizard.md)
- Move Pairing to Channels > Configuration
- Move Bootstrapping to Agents > Fundamentals
- Move macOS app onboarding to Platforms > macOS companion app
- Move Lore to Help > Community
- Remove duplicate install instructions from openclaw.md
- Mirror navigation changes in zh-CN tabs
- No content deleted — all detail preserved or relocated

* docs: move deployment pages to install/, fix Platforms tab routing, clarify onboarding paths

- Move deployment guides (fly, hetzner, gcp, macos-vm, exe-dev, railway, render,
  northflank) from platforms/ and root to install/
- Add 'Hosting and deployment' group to Install tab
- Slim Gateway & Ops 'Remote access and deployment' down to 'Remote access'
- Swap Platforms tab before Gateway & Ops to fix path-prefix routing
- Move macOS app onboarding into First steps (parallel to CLI wizard)
- Rename sidebar titles to 'Onboarding: CLI' / 'Onboarding: macOS App'
- Add redirects for all moved paths
- Update all internal links (en + zh-CN)
- Fix img tag syntax in onboarding.md

docs/zh-CN/platforms/digitalocean.md

@@ -34,7 +34,7 @@ x-i18n:
 **选择提供商：**
 
 - DigitalOcean：最简单的用户体验 + 可预测的设置（本指南）
-- Hetzner：性价比高（参见 [Hetzner 指南](/platforms/hetzner)）
+- Hetzner：性价比高（参见 [Hetzner 指南](/install/hetzner)）
 - Oracle Cloud：可以 $0/月，但更麻烦且仅限 ARM（参见 [Oracle 指南](/platforms/oracle)）
 
 ---
@@ -263,7 +263,7 @@ free -h
 
 ## 另请参阅
 
-- [Hetzner 指南](/platforms/hetzner) — 更便宜、更强大
+- [Hetzner 指南](/install/hetzner) — 更便宜、更强大
 - [Docker 安装](/install/docker) — 容器化设置
 - [Tailscale](/gateway/tailscale) — 安全远程访问
 - [配置](/gateway/configuration) — 完整配置参考