Clarify documentation for code owners group hierarchy

What does this MR do?

Include an additional example to clarify the use of GitLab subgroups in the CODEOWNERS file.

While configuring gitlab-org/gitlab/.gitlab/CODEOWNERS to use the @gitlab-org/secure/threat-insights-backend-team sub-subgroup, I got confused about the hierarchy and had to ask for help in slack (internal link).

The existing documentation only provides an example of using groups in the upper hierarchy. The added example is for using groups in the lower hierarchy.

Related issues

This is the opposite situation addressed by #32432 (closed).

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have developer access or higher (for example, GitLab team members or Core Team members)
  • Apply the documentation label, plus:
    • The corresponding DevOps stage and group label, if applicable.
    • development guidelines when changing docs under doc/development/*, CONTRIBUTING.md, or README.md.
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    • development guidelines and Description templates (.gitlab/*) when creating/updating issue and MR description templates.
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Do not add the feature, frontend, backend, ~"bug", or database labels if you are only updating documentation. These labels will cause the MR to be added to code verification QA issues.

When applicable:

• Update the permissions table.
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• Add/update the feature flag section.
• If you're changing document headings, search doc/*, app/views/*, and ee/app/views/* for old headings replacing with the new ones to avoid broken anchors.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

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3. If there has not been a technical writer review, create an issue for one using the Doc Review template.