Add deploy keys endpoints to OpenApi V2 docs

What does this MR do and why?

This merge request adds endpoints from API::DeployKeys to the OpenApi V2 documentation as per #211512.

Please note that:

• Changes to doc/api/openapi/openapi_v2.yaml were excluded to avoid conflicts within the same documentation effort.
• Errors related to consumes property were ignored as per https://gitlab.com/gitlab-org/gitlab/-/issues/377941.
• Descriptions of all endpoints were updated to match the markdown documentation.

I have also added a couple of questions (1, 2) below. I would appreciate if reviewers let me know their thoughts on these.

Resolves #376896.

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