Provide a document to describe how to reuse the existing Argo CD

KubeSphere DevOps `v3.3.0` be able to reuse the existing Argo CD instance if users don't want to install it again. Please follow these steps:

* Please make sure you have installed Argo CD in the default namespace: `argocd`
* Change the configuration of the ClusterConfiguration aka `cc`
* You can find it from the namespace `kubesphere-system`
* Set `spec.devops.argocd.installation` to be `custom`. See also [details](https://github.com/kubesphere/ks-installer/blob/d9b03bf2c92b59465b3f8b5d2450037812058e09/roles/ks-devops/tasks/main.yaml#L66).
* Please change `.spec.devops.argocd.namespace` to the real namespace of your Argo CD instance if it's not in the default namespace

/area devops

Start by reviewing the referenced ks-installer task and the issue’s ClusterConfiguration steps for DevOps v3.3.0. Locate the appropriate KubeSphere website documentation area, then document the prerequisites, the custom installation setting, and the Argo CD namespace configuration; done means users can follow the page to reuse an existing instance.

Written by the indexing model from the issue text.