Description of the problem, including code/CLI snippet

GitLab has an endpoint for adding merge requests to a merge train: https://docs.gitlab.com/api/merge_trains/#add-a-merge-request-to-a-merge-train

As far as I can tell, this endpoint is not exposed in python-gitlab. The only functionality I can find is to list merge trains in a project: https://python-gitlab.readthedocs.io/en/stable/api/gitlab.v4.html#gitlab.v4.objects.ProjectMergeTrain

If this functionality exists, it needs better documentation or an example.

Expected Behavior

Support adding a merge request to a merge train.

Actual Behavior

N/A

Specifications

• python-gitlab version: 7.1.0
• Gitlab server version (or gitlab.com): 16.11 and possibly older versions also support adding a merge request to a merge train