Separating tasks from Secret concept

There are many contents in the (*very big*) Secret concept page which
are actually tasks. This PR proposes a separation of some contents
into separate tasks, so that we have a (hopefully) better organization
of the content and we make room for improvement to the concept itself.

Note that the creation of the `configmap-secret` directory (instead of
`secret`) is an intent to create a folder for both ConfigMap and Secret,
both of which are about configurations though there are quite some
differences between them.

content/en/docs/tasks/configmap-secret/managing-secret-using-kustomize.md

@@ -0,0 +1,128 @@
+---
+title: Managing Secret using Kustomize
+content_type: task
+weight: 30
+description: Creating Secret objects using kustomization.yaml file.
+---
+
+<!-- overview -->
+
+Since Kubernetes v1.14, `kubectl` supports
+[managing objects using Kustomize](/docs/tasks/manage-kubernetes-objects/kustomization/).
+Kustomize provides resource Generators to create Secrets and ConfigMaps. The
+Kustomize generators should be specified in a `kustomization.yaml` file inside
+a directory. After generating the Secret, you can create the Secret on the API
+server with `kubectl apply`.
+
+## {{% heading "prerequisites" %}}
+
+{{< include "task-tutorial-prereqs.md" >}}
+
+<!-- steps -->
+
+## Create the Kustomization file
+
+You can generate a Secret by defining a `secretGenerator` in a
+`kustomization.yaml` file that references other existing files.
+For example, the following kustomization file references the
+`./username.txt` and the `./password.txt` files:
+
+```yaml
+secretGenerator:
+- name: db-user-pass
+  files:
+  - username.txt
+  - password.txt
+```
+
+You can also define the `secretGenerator` in the `kustomization.yaml`
+file by providing some literals.
+For example, the following `kustomization.yaml` file contains two literals
+for `username` and `password` respectively:
+
+```yaml
+secretGenerator:
+- name: db-user-pass
+  literals:
+  - username=admin
+  - password=1f2d1e2e67df
+```
+
+Note that in both cases, you don't need to base64 encode the values.
+
+## Create the Secret
+
+Apply the directory containing the `kustomization.yaml` to create the Secret.
+
+```shell
+kubectl apply -k .
+```
+
+The output is similar to:
+
+```
+secret/db-user-pass-96mffmfh4k created
+```
+
+Note that when a Secret is generated, the Secret name is created by hashing
+the Secret data and appending the hash value to the name. This ensures that
+a new Secret is generated each time the data is modified.
+
+## Check the Secret created
+
+You can check that the secret was created:
+
+```shell
+kubectl get secrets
+```
+
+The output is similar to:
+
+```
+NAME                             TYPE                                  DATA      AGE
+db-user-pass-96mffmfh4k          Opaque                                2         51s
+```
+
+You can view a description of the secret:
+
+```shell
+kubectl describe secrets/db-user-pass-96mffmfh4k
+```
+
+The output is similar to:
+
+```
+Name:            db-user-pass
+Namespace:       default
+Labels:          <none>
+Annotations:     <none>
+
+Type:            Opaque
+
+Data
+====
+password.txt:    12 bytes
+username.txt:    5 bytes
+```
+
+The commands `kubectl get` and `kubectl describe` avoid showing the contents of a `Secret` by
+default. This is to protect the `Secret` from being exposed accidentally to an onlooker,
+or from being stored in a terminal log.
+To check the actual content of the encoded data, please refer to
+[decoding secret](/docs/tasks/configmap-secret/managing-secret-using-kubectl/#decoding-secret).
+
+## Clean Up
+
+To delete the Secret you have just created:
+
+```shell
+kubectl delete secret db-user-pass-96mffmfh4k
+```
+
+<!-- Optional section; add links to information related to this topic. -->
+## {{% heading "whatsnext" %}}
+
+- Read more about the [Secret concept](/docs/concepts/configuration/secret/)
+- Learn how to [manage Secret with the `kubectl` command](/docs/tasks/configmap-secret/managing-secret-using-kubectl/)
+- Learn how to [manage Secret using config file](/docs/tasks/configmap-secret/managing-secret-using-config-file/)
+