Document how to implement incremental rollouts with GitLab CI

data/release_posts/unreleased/doc_green_blue_deployment.yml

+features:
+  secondary:
+    - name: "Document how to implement incremental rollouts with GitLab CI"
+      available_in: [core, starter, premium, ultimate]
+      documentation_link: 'https://docs.gitlab.com/ee/ci/yaml/#extends'
+      reporter: ogolowinski
+      stage: release
+      categories:
+        - 'incremental Rollout'
+        - 'Continuous Delivery'
+      issue_url: 'https://gitlab.com/gitlab-org/gitlab/issues/27264'
+      description: |
+        Auto DevOps offers an incremental rollout feature which uses GitLab CI. We are happy to instruct Customers who are not using Auto DevOps how to achieve the same functionality.