feat: add interactive AI demo page for GitHub Pages

- New docs/demo.html: Solace-branded interactive chat interface
  powered by Claude via a secure Lambda + API Gateway proxy
- Users describe integration scenarios and get architecture proposals
  with Solace topic taxonomy, schemas, and deployment strategies
- Three pre-built example cards for quick starts
- Markdown rendering, conversation history, rate limiting
- Updated docs/index.html with links to the new demo

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

Author: raphael-solace