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>

Author: JusterZhu