feat(docs): docs viewer, API reference generator, markdown rendering
- Add YAML frontmatter (title/order/category) to 13 docs/*.md files
- Add services/docs_service.py: mtime-cache, frontmatter parsing w/ fallback,
bleach sanitize, path traversal protection
- Add routes/docs.py: GET /api/docs, GET /api/docs/<slug>, GET /api/docs/api-reference
- Register docs_bp in app.py
- Add frontend /docs + /docs/[slug] routes (sidebar nav + markdown render)
- Add frontend/src/types/docs.ts interfaces
- Add link from /help page to /docs viewer
- Add scripts/validate_docs.py: frontmatter + broken link validation
- Add elemes.sh docs-validate command
Endpoints documented in API reference: /quiz-attempts/<lesson>,
/sessions, /bab/<folder>, /compile/sessions, /lesson/<slug>.json + all 30 routes