datasource/gitlab_instance_variable: Add missing doc examples

Author: timofurrer

docs/data-sources/instance_variable.md

@@ -13,7 +13,13 @@ The `gitlab_instance_variable` data source allows to retrieve details about an i
 
 **Upstream API**: [GitLab REST API docs](https://docs.gitlab.com/ee/api/instance_level_ci_variables.html)
 
+## Example Usage
 
+```terraform
+data "gitlab_instance_variable" "foo" {
+  key = "foo"
+}
+```
 
 <!-- schema generated by tfplugindocs -->
 ## Schema

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

@@ -0,0 +1,3 @@
+data "gitlab_instance_variable" "foo" {
+  key = "foo"
+}