Add docs for `prepare` keyword on `environment:action:` keyword

doc/ci/environments/index.md

@@ -811,6 +811,31 @@ stopped environment:
 
 Environments can also be deleted by using the [Environments API](../../api/environments.md#delete-an-environment).
 
+### Prepare an environment
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/208655) in GitLab 13.2.
+
+By default, GitLab creates a [deployment](#viewing-deployment-history) every time a
+build with the specified environment runs. Newer deployments can also
+[cancel older ones](deployment_safety.md#skip-outdated-deployment-jobs).
+
+You may want to specify an environment keyword to
+[protect builds from unauthorized access](protected_environments.md), or to get
+access to [scoped variables](#scoping-environments-with-specs). In these cases,
+you can use the `action: prepare` keyword to ensure deployments won't be created,
+and no builds would be canceled:
+
+```yaml
+build:
+  stage: build
+  script:
+    - echo "Building the app"
+  environment:
+    name: staging
+    action: prepare
+    url: https://staging.example.com
+```
+
 ### Grouping similar environments
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/7015) in GitLab 8.14.