Add CI variable types usage example

Update documentation with example how to use CI variable types.

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/46806.

doc/ci/variables/README.md

@@ -58,8 +58,29 @@ the need to specify the value itself.
 
 There are two types of variables supported by GitLab:
 
-- `env_var`: the runner will create environment variable named same as the variable key and set its value to the variable value.
-- `file`: the runner will write the variable value to a temporary file and set the path to this file as the value of an environment variable named same as the variable key.
+- "Variable": the Runner will create an environment variable named same as the variable key and set its value to the variable value.
+- "File": the Runner will write the variable value to a temporary file and set the path to this file as the value of an environment variable named same as the variable key.
+
+Many tools (like [AWS CLI](https://docs.aws.amazon.com/cli/latest/userguide/cli-configure-envvars.html) and [kubectl](https://kubernetes.io/docs/concepts/configuration/organize-cluster-access-kubeconfig/#the-kubeconfig-environment-variable)) provide the ability to customise configuration using files by either providing the file path as a command line argument or an environment variable. Prior to the introduction of variable types, the common pattern was to use the value of a CI variable, save it in a file, and then use the newly created file in your script:
+
+```bash
+# Save the content of variable in a file
+echo "$KUBE_CA_PEM" > "$(pwd)/kube.ca.pem"
+ # Use the newly created file
+kubectl config set-cluster e2e --server="$KUBE_URL" --certificate-authority="$(pwd)/kube.ca.pem"
+```
+
+This can be simplified by creating a variable of type "File" and using it directly. For example, let's say we have the following variables.
+
+![CI/CD settings - variable types usage example](img/variable_types_usage_example.png)
+
+We can then call them from `.gitlab-ci.yml` like this:
+
+```bash
+kubectl config set-cluster e2e --server="$KUBE_URL" --certificate-authority="$KUBE_CA_PEM"
+```
+
+Variable types can be set via the [UI](#via-the-ui) or the [API](../../api/project_level_variables.md#create-variable), but not in `.gitlab-ci.yml`.
 
 #### Masked variables