Documentation for "Show License Management reports at pipeline level"

Documentation for https://gitlab.com/gitlab-org/gitlab-ee/issues/5491.

We can do add something in https://docs.gitlab.com/ee/user/project/merge_requests/license_management.html like we have in https://docs.gitlab.com/ee/user/project/merge_requests/sast.html#security-report-under-pipelines.

We should also add that only Maintainers can approve or blacklist licenses.