Sync Agents SDK documentation from PR 812

This sync includes comprehensive documentation for the Agents SDK covering:

- **Quick Start Guide** (getting-started/quick-start.mdx) - Installation, first agent, state basics, and deployment
- **Routing** (concepts/routing.mdx) - URL patterns, name resolution, custom routing, and server-sent identity
- **Callable Methods** (api-reference/callable-methods.mdx) - @callable decorator, RPC over WebSocket, and streaming responses
- **Queue System** (api-reference/queue-system.mdx) - Background task queue with queue(), dequeue(), and related APIs
- **Add to Existing Project** (guides/add-to-existing-project.mdx) - Integration guide for adding agents to existing Workers
- **Email Routing** (guides/email-routing.mdx) - Email routing with routeAgentEmail(), resolvers, and secure reply handling
- **Observability** (platform/observability.mdx) - Tracing and monitoring agent activity

All documentation follows Cloudflare docs style guidelines:
- MDX format with proper frontmatter
- TypeScriptExample, WranglerConfig, and PackageManagers components
- No contractions, professional tone
- Absolute paths for internal links with trailing slashes

Source: cloudflare/agents PR 812
cloudflare/agents#812

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: github-actions[bot]