docs: refresh README DITA section and sync metrics to v23.0

- Replace single-link DITA section with structured two-document-set listing
  (Technical Architecture Guide 23 chapters + User Guide 12 chapters)
- Remove stale `preview_docs.py` reference
- Update metrics: 40 Toolsets, 270+ tools, 280 REST APIs, 3588 tests,
  app.py 3948 lines, frontend_api.py 3438 lines + 24 route modules
- Sync both Chinese and English READMEs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Gemini CLI