Add types and examples to merge_request related entities for OpenAPI docs
We've added MergeRequest to OpenAPI in !104339 (merged).
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.