terragrunt: add page (#7619)

pages/common/terragrunt.md

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+# terragrunt
+
+> Keep your Terraform CLI arguments DRY.
+> More information: <https://terragrunt.gruntwork.io>.
+
+- Generate and show an execution plan:
+
+`terragrunt plan`
+
+- Build or change infrastructure:
+
+`terragrunt apply`
+
+- Show current deployment (from state):
+
+`terragrunt show`
+
+- Show module output values:
+
+`terragrunt output`
+
+- Destroy Terraform-managed infrastructure:
+
+`terragrunt destroy`
+
+- Build or change infrastructure from a tree of Terragrunt modules (stack):
+
+`terragrunt run-all apply`