OpenAPI page: minor updates, and change YAML file link

What does this MR do?

This MR updates a link in doc/api/openapi/openapi_interactive.md, and blends a couple of subheadings together into a concept area. (This page didn't really have one.)

How we got to this point is worth outlining, because it's a bit of a ride.

• As part of Inconsistent use of 'cherry-pick' versus 'cherr... (gitlab-docs#1849 - closed), I tried to update strings that used cherry pick instead of cherry-pick.
• One of the findings that needed fixing was doc/api/openapi/openapi_v2.yaml auto-generated from Rake task bin/rake gitlab:openapi:generate, which led me to create Regenerate API file, document how it was done (!179892 - merged) to update openapi_v2.yaml.
• When I had trouble identifying an owner for the YAML file, that led me to the giant epic Document the REST API in OpenAPI V2 (&8926). I added &8926 (comment 2332786567).
• As part of the discussion, @m_frankiewicz asked that we update the link in doc/api/openapi/openapi_interactive.md.

And that's why I'm updating a link in doc/api/openapi/openapi_interactive.md, in a part of the docs I'm never in, to update the link to a YAML file that I just wanted to change one character in. Good times! 🎉

I'll send this over to @ashrafkhamis because this page belongs to groupimport and integrate, and he will be FAR more familiar with that area than I am.

Related issues

• Related to Regenerate API file, document how it was done (!179892 - merged)
• Related to Inconsistent use of 'cherry-pick' versus 'cherr... (gitlab-docs#1849 - closed)
• Related to Document the REST API in OpenAPI V2 (&8926) (epic)

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
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• ~"frontend"
• ~"backend"
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These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

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