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
- Custom resource structure and relationships
- Error handling and status transitions
- Multi-repo workflow
- Monitoring and observability
- Security architecture

Update mkdocs.yml navigation to include Architecture section with
links to platform-architecture.md and amber-workflow.md for better
discoverability in the documentation site.

Fixes #441

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

Author: Amber Agent