doc(CU-869azd0ae): update meaning of system role

[eliecharra]

Description of the change

Follow up of https://spacelift-io.slack.com/archives/C09C7LS8K16/p1761647374700239

Checklist

Please make sure that the proposed change checks all the boxes below before requesting a review:

• I have reviewed the guidelines for contributing to this repository.
• The preview looks fine.
• The tests pass.
• The commit history is clean and meaningful.
• The pull request is opened against the main branch.
• The pull request is no longer marked as a draft.
• You agree to license your contribution under the MIT license to Spacelift (not required for Spacelift employees).
• You have updated the navigation files correctly:
  • No new pages have been added, or;
  • Only nav.yaml has been updated because the changes only apply to SaaS, or;
  • Only nav.self-hosted.yaml has been updated because the changes only apply to Self-Hosted, or;
  • Both nav.yaml and nav.self-hosted.yaml have been updated.

If the proposed change is ready to be merged, please request a review from @spacelift-io/solutions-engineering. Someone will review and merge the pull request.

Spacelift employees should request reviews from the relevant engineers and are allowed to merge pull requests after they got at least one approval.

Thank you for your contribution! 🙇