feat: Add comprehensive Workflows integration documentation

Syncs documentation from cloudflare/agents PR #799 which adds seamless
integration between Cloudflare Agents and Cloudflare Workflows for
durable, multi-step background processing.

This update replaces the basic workflow triggering documentation with
comprehensive coverage of the new AgentWorkflow base class, including:

- AgentWorkflow class with typed Agent access
- Workflow tracking in Agent SQLite database
- Bidirectional communication between Agents and Workflows
- Lifecycle callbacks (onWorkflowProgress, onWorkflowComplete, etc.)
- Approval methods for human-in-the-loop flows
- Common patterns (background processing, approvals, retries, state sync)
- Custom progress types
- Best practices for cleanup and migration

Related PR: cloudflare/agents#799

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

Author: github-actions[bot]