docs: rewrite Beginner cookbook with step-by-step self-guided flow (#91)

* docs: rewrite Beginner cookbook with step-by-step self-guided flow

- Replace Hub-centric 'run the samples' approach with self-guided
  'write-your-own' flow: Client → Upgrade → Tools → Server → verify
- Add bilibili video embed (BV12P9dBiEEh) and ASCII update flow diagram
- Client code: only 3 secrets in UpdateRequest — all identity fields
  (MainAppName, ClientVersion, UpdateAppName, etc.) are auto-discovered
  by AppMetadataDiscoverer from generalupdate.manifest.json
- Upgrade code: AppType.Upgrade, no SetConfig — params via encrypted IPC
- Phase 4 (Tools): use Config tab, not Patch tab; steps match actual UI
  (.csproj path → Analyze → Generate Sample)
- Fix directory structure: Upgrade.exe lives in update/ subdirectory per
  manifest's updatePath, not alongside Client
- Manifest JSON example reflects actual ManifestInfo field names and
  omits secrets (they belong in code)
- Sync all three locales: zh (default), zh-Hans, en

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* fix: replace react-player import with bilibili iframe embed

react-player is not a project dependency, causing Docusaurus build
to fail with 'Module not found'. Use bilibili's native iframe embed
instead, which requires no extra npm package.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* fix: address Copilot review — diagram endpoint path and curl -X POST

- ASCII diagram: POST /Verification -> POST /Upgrade/Verification
- Troubleshooting table: add missing -X POST to curl command

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: JusterZhu