Replace the Users of website to user docs

As we have renew a user docs in https://huaweicse.github.io/servicecomb-java-chassis-doc/.

I suggest to replace the user part of website to this docs, my idea is:

1. Create several items in http://servicecomb.incubator.apache.org/users/ , each item corresponds to a project, such as java-chassis, saga, servicecenter
2.  Change https://huaweicse.github.io/servicecomb-java-chassis-doc/ to https://servicecomb.incubator.apache,org/servicecomb-java-chassis-doc/
3. link  https://servicecomb.incubator.apache,org/servicecomb-java-chassis-doc/ to the Java-chassis item under http://servicecomb.incubator.apache.org/users/

If we do it like this way , we also should change user docs of the saga and servicecenter to gitbook to keep the user experience consistent.