datasource/gitlab_instance_variables: Add missing doc examples

Author: timofurrer

docs/data-sources/instance_variables.md

@@ -13,7 +13,11 @@ The `gitlab_instance_variables` data source allows to retrieve all instance-leve
 
 **Upstream API**: [GitLab REST API docs](https://docs.gitlab.com/ee/api/instance_level_ci_variables.html)
 
+## Example Usage
 
+```terraform
+data "gitlab_instance_variables" "vars" {}
+```
 
 <!-- schema generated by tfplugindocs -->
 ## Schema

examples/data-sources/gitlab_instance_variables/data-source.tf

@@ -0,0 +1 @@
+data "gitlab_instance_variables" "vars" {}