Adds JPG files (in doc dir) to CODEOWNERS

What does this MR do?

As part of reviewing Add documentation for Kibana Error budgets dash... (!114416 - merged), I approved, but my approval wasn't enough. That was odd, because both files were in the doc/development/ tree.

My thought process:

1. Was it age? If yes, a rebase would have fixed it. (It didn't.)
2. Was it content? I stared at the file listing for a moment and zeroed in on the JPG file. We don't use those often.

Sure enough, we forgot to declare JPG images in the doc/ directory as being approvable by technical writers:

## Files that are excluded from required approval
/.gitlab/issue_templates/*.md
/.gitlab/merge_request_templates/*.md
/doc/*.md
/doc/**/*.md
/doc/**/*.png
/data/deprecations/*.yml
/data/removals/**/*.yml

Let's fix this issue. To me it's an obvious oversight. Adding devopscreate and groupsource code because I was working a merge request for them when I stumbled into this problem.

Related issues

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
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If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

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If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

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