docs: add comprehensive platform architecture diagrams

Add detailed architecture documentation with Mermaid diagrams covering:
- System overview and component architecture
- Agentic session lifecycle and data flows
- Authentication and authorization patterns
- Multi-tenancy architecture
- Development and production deployment topologies
- Network topology and communication protocols

Update README.md to reference the new architecture documentation
in the Support & Documentation section.

Add Architecture section to mkdocs.yml navigation for better
discoverability in the documentation site.

Fixes #438

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: Amber Agent

README.md

@@ -550,7 +550,8 @@ See [e2e/README.md](e2e/README.md) for detailed documentation, troubleshooting,
 ### Deployment & Configuration
 - **Deployment Guide**: [docs/OPENSHIFT_DEPLOY.md](docs/OPENSHIFT_DEPLOY.md)
 - **OAuth Setup**: [docs/OPENSHIFT_OAUTH.md](docs/OPENSHIFT_OAUTH.md)
-- **Architecture Details**: [diagrams/](diagrams/)
+- **Platform Architecture**: [docs/diagrams/platform-architecture.md](docs/diagrams/platform-architecture.md) - System components, data flows, deployment architecture
+- **Amber Workflows**: [docs/diagrams/amber-workflow.md](docs/diagrams/amber-workflow.md) - Issue-to-PR automation workflows
 - **API Documentation**: Available in web interface after deployment
 
 ### GitLab Integration