Set RocketBot as primary UI in doc (#2524)

* Set RocketBot as primary UI in doc

* Update submodule.

docs/en/guides/README.md

@@ -48,10 +48,12 @@ want to push upstream back to our Apache OSS repo, please read these principles.
 ### UI developer
 Our UI is constituted by static pages and web container.
 
-- **Static pages** is built based on [Ant Design Pro](https://pro.ant.design/), which source codes are 
-hosted in our [UI repository](https://github.com/apache/incubator-skywalking-ui).
+- [RocketBot UI](https://github.com/apache/skywalking-rocketbot-ui) is SkyWalking primary UI since 6.1 release.
+It is built with vue + typescript. You could know more at the rocketbot repository.
 - **Web container** source codes are in `apm-webapp` module. This is a just an easy zuul proxy to host
 static resources and send GraphQL query requests to backend.
+- [Legacy UI repository](https://github.com/apache/incubator-skywalking-ui) is still there, but not included
+in SkyWalking release, after 6.0.0-GA.
 
 ## For release
 [Apache Release Guide](How-to-release.md) introduces to the committer team about doing official Apache version release, to avoid 

docs/en/setup/backend/ui-setup.md

@@ -31,8 +31,3 @@ security:
 
 ```
 
-## Another Open Source UI RocketBot
-
-A modern and awesome UI for Apache SkyWalking. 
-
-Github: [https://github.com/TinyAllen/rocketbot](https://github.com/TinyAllen/rocketbot)

skywalking-ui @ 116d6d8f

-Subproject commit 0a67a4cb2f99e7ea2910cb08738190b4fa5aef7f
+Subproject commit 116d6d8f6c65966da3cc26fcfac97879e30fdad2