docs: add quickstart page, restructure docs and sidebar
Summary
- New /docs/quickstart page: 5-minute setup for end users to install their first Skill
- Installation guide now focuses on platform deployment (Docker/manual)
- Usage guide restructured as comprehensive end-user tutorial with full Skill catalog
- DocsSidebar widened to w-64 with increased item spacing
Why
The previous /docs/quickstart route was a 404. Documentation responsibilities were unclear — installation guide mixed platform deployment instructions with end-user Skill installation commands. This restructure gives each doc a clear audience and purpose.
Changes
- New: src/app/docs/quickstart/page.tsx
- Modified: src/app/docs/installation/page.tsx
- Modified: src/app/docs/usage/page.tsx
- Modified: src/components/docs/DocsSidebar.tsx