Document the "Review-response" SLO in the handbook

Why is this change being made?

This consolidates the "Review-response" SLO to the handbook since it's company-specific, instead of https://docs.gitlab.com/ee/development/code_review.html#review-turnaround-time.

For reference, the relevant past updates to this page are:

• First-response SLO introduction for community MRs: !20165 (merged)
• First-response SLO update to apply this to all MRs: !37137 (merged)
• Review-response SLO introduction (not mentioning if that was for team-authored or all MRs): gitlab-org/gitlab!22035 (merged)

Today, the First-response SLO tends to duplicate the Review-response SLO, so only keeping the latter makes sense. This is also in accordance with an old discussion which happened in #5972 (comment 269720244).

Lastly, this MR mentions a different SLO for Resident contributor MRs (which isn't officially documented for now, so maybe we shouldn't introduce it in this MR until we document the concept?). <- We're moving this to a new MR (!109710 (merged))

Corresponding gitlab-org/gitlab MR is at gitlab-org/gitlab!95188 (merged).

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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