Restructure Code Review runbook per by service/feature

Context

We received feedback about runbooks needing to be structured by service rather than by group.

The goal is to help SREs quickly route work and troubleshoot issues. When they encounter errors like "Workhorse is not working," they need to find the Workhorse runbook directly, rather than searching through the SCM runbook to find the relevant information.

In groupcode review's case, we can split by features rather than by service, since we do not own any specific service (like Workhorse).

Proposal

Split the content of https://gitlab.com/gitlab-com/runbooks/-/blob/master/docs/uncategorized/code-review-group.md into 2 separate runbooks: one for Merge Requests, and one for the GitLab CLI.

Add links to the 3 playbooks we have:

• GitLab CLI: https://internal.gitlab.com/handbook/engineering/tier2-oncall/playbooks/create/cli/
• Merge Request Diffs: https://internal.gitlab.com/handbook/engineering/tier2-oncall/playbooks/create/merge-request-diffs/
• Mergeability: https://internal.gitlab.com/handbook/engineering/tier2-oncall/playbooks/create/mergeability/