Documentation: "Run Pipeline" button in "Pipelines" tab in merge requests

It seems we don't have documentation for the "Run Pipeline" button in "Pipelines" tab in merge requests (i.e. a button for creating merge request pipelines).

It's helpful if we have documentation with a screenshot https://docs.gitlab.com/ee/ci/merge_request_pipelines/, as I want to point it to users in #11934 (closed).