datasource/gitlab_group_variable: Add missing doc examples

Author: timofurrer

docs/data-sources/group_variable.md

@@ -13,7 +13,21 @@ The `gitlab_group_variable` data source allows to retrieve details about a group
 
 **Upstream API**: [GitLab REST API docs](https://docs.gitlab.com/ee/api/group_level_variables.html)
 
-
+## Example Usage
+
+```terraform
+data "gitlab_group_variable" "foo" {
+  group = "my/example/group"
+  key   = "foo"
+}
+
+# Using an environment scope
+data "gitlab_group_variable" "bar" {
+  group             = "my/example/group"
+  key               = "bar"
+  environment_scope = "staging/*"
+}
+```
 
 <!-- schema generated by tfplugindocs -->
 ## Schema

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

@@ -0,0 +1,11 @@
+data "gitlab_group_variable" "foo" {
+  group = "my/example/group"
+  key   = "foo"
+}
+
+# Using an environment scope
+data "gitlab_group_variable" "bar" {
+  group             = "my/example/group"
+  key               = "bar"
+  environment_scope = "staging/*"
+}