docs: consolidate website redesign plan and remove stale docs

- Remove docs/cf-rest-api-integration-plan.md (implemented in 9d8d792)
- Add docs/designs/website-final-plan.md (reviewed, 3 rounds of fixes)
- Fix: section ordering (how-it-works before FAQ)
- Fix: merge platforms into use-cases section
- Fix: Schema FAQ uses i18n source (not hardcoded placeholder)
- Fix: robots.txt unified rules (no AI crawler override)
- Fix: MCP config examples (Claude Desktop JSON + Cursor)
- Fix: CF WAF tradeoff documented
- Fix: SVG icons instead of emoji

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Digidai