Docs: improvement of the `dubboctl` quick start guide

[yin1999]

The steps listed in the dubboctl quick start guide is not corrected. The problems found in testing are as follows:

• [ ] The command to init the demo environment is not working:

  dubboctl manifest install --profile=demo

  The command should be changed to dubboctl manifest install --set profile=demo

  A fix is created to resolve the services creation problem: #290

• [ ] Missing the step to create k8s namespace dubbo-demo

• [ ] The branch name to dubbo-samples specified in this article is not correct main -> master.

• [ ] The command (dubboctl build --dockerfile=./Dockerfile) used to build the application is not correct, it should be replaced with:

  dubboctl build --useDockerfile

• [ ] The command used to generate the deploy should specify the namespace: dubboctl deploy --namespace dubbo-demo.

• [x] The port field of the Service resource within kube.yaml file generated by dubboctl deploy is missing (name is missing when generate multiple ports). #304

[yin1999]

cross ref: #306, which introduced support for installing dubbo-admin as a k8s service