Sync docs: Add Agent System documentation from PR #675

Add comprehensive documentation for the new Agent System (agents/sys),
a multi-agent orchestration framework for building complex AI systems.

Documentation includes:
- Overview of Agent System capabilities and architecture
- Getting started guide with setup instructions
- Detailed architecture documentation
- Complete API reference

Synced from: cloudflare/agents#675

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

Author: github-actions[bot]