Add a complete documentation site under docs-site/ using Docusaurus 3,
with Chinese (default) and English translations. Includes user guide
(SSH, SFTP, terminal, quick commands, themes, translation, security,
shortcuts), developer docs (architecture, setup, frontend, backend,
contributing), FAQ, and a custom landing page.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>