docs: reorganize README into focused docs/ files

README trimmed to a lean landing page. Detailed content moved to:
- docs/ARCHITECTURE.md (BFF pattern, ReAct agent, security model)
- docs/INFRASTRUCTURE.md (Cloud topology, Terraform, Secret Manager, CI/CD)
- docs/AZURE_SETUP.md (Entra ID registration, Cosmos DB, frontend config)
- docs/DEVELOPMENT.md (local setup, testing, code style)

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

Author: ChingEnLin