docs: fill in docs page for managing clusters

docs/basics/clusters/manage.md

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+# Managing Clusters
+
+## Using the `argocd` CLI
+
+### Listing all clusters
+
+To list all clusters, run `argocd cluster list`. You should see a list of output with details about the clusters.
+
+```bash
+$ argocd cluster list
+SERVER                          NAME        VERSION  STATUS      MESSAGE
+https://kubernetes.default.svc  in-cluster  1.16+    Successful  
+```
+
+### Get details about a cluster
+
+To get more detailed information about a specific cluster, you can use `argocd cluster get <server>`, where `server` is the server URL of the cluster.
+
+```bash
+$ argocd cluster get https://kubernetes.default.svc
+config:
+  tlsClientConfig:
+    insecure: false
+connectionState:
+  attemptedAt: "2021-03-19T16:42:09Z"
+  message: ""
+  status: Successful
+info:
+  applicationsCount: 6
+  cacheInfo:
+    apisCount: 54
+    lastCacheSyncTime: "2021-03-19T16:39:36Z"
+    resourcesCount: 504
+  connectionState:
+    attemptedAt: "2021-03-19T16:42:09Z"
+    message: ""
+    status: Successful
+  serverVersion: 1.16+
+name: in-cluster
+server: https://kubernetes.default.svc
+serverVersion: 1.16+
+```
+
+### Add a cluster
+
+To add a cluster using the CLI:
+
+1. Ensure you have a valid context in your kubeconfig for the cluster. Running `argocd cluster add` will list all available contexts.
+2. Add the cluster with `argocd cluster add <context name>`.
+
+Adding a cluster with `argocd cluster add` installs a ServiceAccount named `argocd-manager` into the `kube-system` namespace of that context and binds the service account to an admin-level ClusterRole. Argo CD uses this service account token to perform its management tasks (i.e. deploy/monitoring).
+
+!!! Tip
+    To register the `in-cluster` cluster as a cluster with its own secret, run
+    `argocd cluster add <cluster name> --in-cluster`
+
+### Removing a cluster
+
+To remove a cluster using the CLI:
+
+1. Identify the server URL for the cluster. Running `argocd cluster list` will show a list of all clusters with their name and server.
+2. Remove the cluster with `argocd cluster rm <server>`
+
+!!! Note
+    Removing a cluster will not remove the Applications associated with that cluster
+
+## Using the web UI
+
+### Get details about a cluster
+
+To view details about a cluster in the web UI, first go to `/settings/clusters/` in your Argo CD instance in your browser, then click on the row for a cluster. You can also click on the "edit" button in the top right corner to edit the cluster name or allowed namespaces.
+
+### Removing a cluster
+
+To remove a cluster using the web UI, first go to `/settings/clusters` in your Argo CD instance in your browser
+Then, find the cluster and click on the three dots on the right hand side of the cluster row, then click "delete".
+
+![Screenshot showing a cluster row in the ArgoCD UI with the "remove" option visible](../../assets/cluster-remove-ui.png)
+
+## Using the kubernetes API
+
+Clusters are stored as kubernetes secrets, so it is possible (but not typically recommended) to manipulate them using the kubernetes API. 
+
+### Listing all clusters
+
+To view all cluster secrets, you can run `kubectl get secret -l argocd.argoproj.io/secret-type="cluster"` in the namespace for your Argo CD instance. The secrets should be in the format `cluster-<server url>-<hash>`, and have keys for `config`, `name`, `server`, and (optionally) `shard`.
+
+```bash
+$ kubectl get secret -l argocd.argoproj.io/secret-type="cluster" -n argocd
+NAME                                        TYPE     DATA   AGE
+cluster-kubernetes.default.svc-3396314289   Opaque   3      3s
+```
+
+```bash
+$ kubectl get secret -n argocd cluster-kubernetes.default.svc-3396314289 -o yaml
+apiVersion: v1
+data:
+  config: ****
+  name: ZG9ja2VyLWRlc2t0b3A=
+  server: aHR0cHM6Ly9rdWJlcm5ldGVzLmRlZmF1bHQuc3Zj
+kind: Secret
+metadata:
+  annotations:
+    managed-by: argocd.argoproj.io
+  creationTimestamp: "2021-03-19T16:59:50Z"
+  labels:
+    argocd.argoproj.io/secret-type: cluster
+  name: cluster-kubernetes.default.svc-3396314289
+  namespace: argocd
+  resourceVersion: "8980461"
+  selfLink: /api/v1/namespaces/argocd/secrets/cluster-kubernetes.default.svc-3396314289
+  uid: 19b453ce-93e7-41f0-b59d-0c4e3b51f3a0
+```
+
+### Removing a cluster
+
+To remove a cluster, identify the name of the cluster secret and run `kubectl delete secret <cluster secret name>`.