docs: update GitLab links

| `--pipelines-keep-per-mr`           | `$PIPELINES_KEEP_PER_MR`       | number of pipelines to keep per merge request (⚠ MR MUST still exist)                          |

| `--pipelines-delete-older-than`     | `$PIPELINES_DELETE_OLDER_THAN` | max age (in days) after which pipelines are deleted (unless they are kept by a keep rule)      |

Cleanup is following the same principle as GitLab's cleanup policy : [GitLab's cleanup policy](https://docs.gitlab.com/ee/user/packages/container_registry/reduce_container_registry_storage.html#cleanup-policy)

Cleanup is following the same principle as GitLab's cleanup policy : [GitLab's cleanup policy](https://docs.gitlab.com/user/packages/container_registry/reduce_container_registry_storage/#cleanup-policy)

The cleanup policy:

from gitlab_butler.main import to_url

# project details : https://docs.gitlab.com/ee/api/projects.html#get-single-project

# project details : https://docs.gitlab.com/api/projects/#get-single-project

def mock_empty_project(project_id: str, archived: bool = False, project_access: int | None = 30, group_access: int | None = 30) -> None:

    permission = dict()

    permission['project_access'] = { 'access_level': project_access } if project_access is not None else None

    return pipelines

# project details : https://docs.gitlab.com/ee/api/projects.html#get-single-project

# project details : https://docs.gitlab.com/api/projects/#get-single-project

def mock_empty_project(project_id: str, branches: list[object] | None = None, tags: list[object] | None = None, merge_requests: list[object] | None = None, pipelines: list[dict[str, object]] = []) -> None:

    permission = dict()

    permission['project_access'] = {'access_level': 30}