Documentation for Incremental Rollouts
We need documentation to describe Incremental Rollouts: https://gitlab.com/gitlab-org/gitlab-ee/issues/5415
Documentation should state how to manually enable incremental rollouts with variables, and how to use them. Even if the feature is available in Core, we should document it as a GitLab Premium feature (see https://gitlab.com/gitlab-org/gitlab-ee/issues/5415#note_71663732 for more details).