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.
issue