docs: integrate mdbook for architecture documentation

[crrow]

Summary

Integrate mdbook to provide comprehensive architecture documentation for the kotoba project.

Acceptance Criteria

• book.toml configured at project root
• mdbook source in docs/book/src/
• Architecture overview chapter covering all modules
• Data flow diagrams (vocabulary learning, TTS synthesis)
• Module relationship documentation
• CI or script to build the book
• Existing docs/guides linked or incorporated