Add response codes and fields to GET /merge_requests

What does this MR do?

Begins work on https://docs.gitlab.com/api/merge_requests/ by tackling the first endpoint, GET /merge_requests: https://docs.gitlab.com/api/merge_requests/#list-merge-requests. This endpoint needs a list of response codes, and a table of fields in the response.

For the technical review: this MR requires more careful review than most MRs I send you, because I'm not able to fully verify the information I'm proposing here. I started digging in lib/api/merge_requests.rb and got tangled up in the dependencies. I tried querying Duo; see internal comment below for the raw response. I saw it pull information from these files, which made me feel better about the dependency chain:

See full file list

• lib/api/entities/merge_request_basic.rb
• lib/api/entities/merge_request_simple.rb
• lib/api/entities/milestone.rb
• lib/api/entities/issuable_entity.rb
• lib/api/entities/user_basic.rb
• lib/api/entities/user_safe.rb
• ee/lib/ee/api/entities/merge_request_basic.rb

What I need from a technical review:

• Are the response codes complete and correct?
• Are the response fields complete and correct?
  • Any fields missing?
  • Are the types and descriptions correct?

I'll have another TW review my language, so don't worry about the wording. Focus on fact-checking me.

Related issues

Related to Fix missing data / structure in Merge Request A... (#433318)

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