docs(installation): rewrite with bare metal + Docker tabs

- Two-tab layout: Bare Metal (VPS/Hackathon) vs Docker
- One-liner with --bare / --docker flags
- Manual install steps without openclaw onboard dependency
- Troubleshooting covers GATEWAY_TOKEN / .env.local issue

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: danielgorgonha