docs: Polish SkyWalking doc and a tiny thing about Zipkin (#4339)

Co-authored-by: 琚致远 <juzhiyuan@apache.org>
Co-authored-by: Shuyang Wu <wosoyoung@gmail.com>

README.md

@@ -116,7 +116,8 @@ A/B testing, canary release, blue-green deployment, limit rate, defense against
 
 - **OPS friendly**
 
-  - OpenTracing: support [Apache Skywalking](docs/en/latest/plugins/skywalking.md) and [Zipkin](docs/en/latest/plugins/zipkin.md)
+  - Zipkin tracing: [Zipkin](docs/en/latest/plugins/zipkin.md)
+  - Open source APM: support [Apache SkyWalking](docs/en/latest/plugins/skywalking.md)
   - works with external service discovery：In addition to the built-in etcd, it also supports [Consul](docs/en/latest/discovery/consul_kv.md) and [Nacos](docs/en/latest/discovery/nacos.md), and [Eureka](docs/en/latest/discovery.md)
   - Monitoring And Metrics: [Prometheus](docs/en/latest/plugins/prometheus.md)
   - Clustering: APISIX nodes are stateless, creates clustering of the configuration center, please refer to [etcd Clustering Guide](https://etcd.io/docs/v3.4.0/op-guide/clustering/).

docs/en/latest/plugins/skywalking.md

@@ -33,7 +33,7 @@ title: skywalking
 
 ## Name
 
-[**Skywalking**](https://github.com/apache/skywalking) is an OpenTracing plugin.\
+[**Skywalking**](https://github.com/apache/skywalking) uses its native Nginx LUA tracer to provide tracing, topology analysis, and metrics from service and URI perspective.\
 The skywalking server can supports both http and grpc protocols. The APISIX client only support http protocols.
 
 ## Attributes

docs/zh/latest/plugins/skywalking.md

@@ -33,7 +33,7 @@ title: skywalking
 
 ## 名字
 
-`Skywalking`(https://github.com/apache/skywalking) 是一个开源的服务跟踪插件。
+`Skywalking`(https://github.com/apache/skywalking) 使用其原生的 Nginx LUA tracer，提供分布式追踪、拓扑分析以及服务和 URI 级别的 metrics 信息。
 
 服务端目前支持http和grpc两种协议，在apisix中目前只支持http协议