Add orphaned CI and flags topics to global nav

This MR adds the Feature flags API topic (feature flags topics in development section), and two CI/CD topics to the global nav.

• https://docs.gitlab.com/ee/api/features.html

• https://docs.gitlab.com/ee/ci/cloud_deployment/ecs/quick_start_guide.html

• https://docs.gitlab.com/ee/ci/resource_groups/index.html

• Link to the merge request of the new page:

• Give the MR a descriptive title.

• Link to the merge request that introduced the new page.