feat: MCPServer resource documentation

## MCPServer Documentation

Documentation for the MCPServer resource introduced in Dapr 1.18.

### Scope
- **How-to guide**: Declare MCPServer resource, list tools, call tools via workflow API
- **MCPServer spec reference**: All fields (endpoint, auth, middleware, catalog)
- **Concept doc**: MCP Server connectivity overview, transport types, durable execution model
- **Middleware hooks**: beforeCallTool/afterCallTool, mutate semantics, authz gates
- **Auth docs**: OAuth2 client credentials flow, SPIFFE JWT injection
- **Feature flags page**: Add MCPServerResource preview feature

### Blocked by
- dapr/dapr#9731 (MCPServer in-process workflow engine)

### Related
- dapr/dapr#9685 (CRD plumbing - merged)
- dapr/proposals#100 (proposal - merged)