Add types and examples to merge_request related entities for OpenAPI docs
We've added MergeRequest to OpenAPI in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/104339. Since the MR was getting too big and also it wasn't a strict requirement, I didn't update the following entities. * Entities::MergeRequestBasic * Entities::MergeRequest * Entities::MergeRequestReviewer * Entities::MergeRequestChanges * Entities::MRNote I think we should consider adding `type` and `example` to these entities similar to https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/api/entities/commit.rb.
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