docs: Add architecture diagrams with Mermaid visualizations

[darshjme]

Summary

• Adds visual architecture documentation with Mermaid diagrams to docs/design/
• Covers standalone and distributed runtime architecture, agent type hierarchy, publish-subscribe message flow, AgentChat team execution, team pattern comparison, and worker protocol lifecycle
• Updates docs/design/readme.md with a table of contents linking all design documents

Motivation

Visual diagrams significantly reduce onboarding time for new contributors and users trying to understand AutoGen's multi-agent architecture. The existing design documents (01 through 05) are text-only. This adds a companion visual reference using Mermaid, which renders natively on GitHub without external tooling.

Changes

• docs/design/06 - Architecture Diagrams.md — new file with 6 Mermaid diagrams
• docs/design/readme.md — added table of contents for all design docs
• Documentation only, no code changes

Diagrams included

1. Standalone Runtime — message router, subscription table, agent catalog
2. Distributed Runtime — host servicer, gateway, worker processes
3. Agent Type Hierarchy — class diagram from core Agent through AgentChat agents and teams
4. Publish-Subscribe Message Flow — sequence diagram showing the code-generation example from the docs
5. AgentChat Team Execution — sequence diagram of GroupChatManager orchestration loop
6. Team Pattern Comparison — side-by-side of RoundRobin, Selector, Swarm, and GraphFlow
7. Worker Protocol Lifecycle — state machine for worker-host communication phases

[darshjme]

@microsoft-github-policy-service agree