Observability Tasks: Link Tracing Use Cases and Documentation（链路追踪使用案例与文档建设）

- [ ] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.

## Describe the proposal

En:As a framework layer middleware, Dubbo's link tracking system function will not be coupled with other link tracking systems. We need to provide common and popular use cases of link tracking systems. Of course, documentation is also indispensable.

中文：Dubbo作为框架层中间件，链路追踪系统功能不会与其他链路追踪系统耦合，需要我们提供常见的、流行的链路追踪系统的使用案例，当然文档也是不可或缺的。