Add job artifacts API to OpenAPIv2

What does this MR do and why?

Add job artifacts API to OpenAPIv2 documentation.

Descriptions and parameters are updated to be conistent with https://docs.gitlab.com/ee/api/job_artifacts.html.

Screenshots or screen recordings

Example OpenAPIv2 containing only job artifacts API can be found in this snippet.

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #376873