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/platforms/digitalocean.md

@@ -27,7 +27,7 @@ If you want a $0/month option and don’t mind ARM + provider-specific setup, se
 **Picking a provider:**
 
 - DigitalOcean: simplest UX + predictable setup (this guide)
-- Hetzner: good price/perf (see [Hetzner guide](/platforms/hetzner))
+- Hetzner: good price/perf (see [Hetzner guide](/install/hetzner))
 - Oracle Cloud: can be $0/month, but is more finicky and ARM-only (see [Oracle guide](/platforms/oracle))
 
 ---
@@ -256,7 +256,7 @@ free -h
 
 ## See Also
 
-- [Hetzner guide](/platforms/hetzner) — cheaper, more powerful
+- [Hetzner guide](/install/hetzner) — cheaper, more powerful
 - [Docker install](/install/docker) — containerized setup
 - [Tailscale](/gateway/tailscale) — secure remote access
 - [Configuration](/gateway/configuration) — full config reference