Initial checklist for Partial Issue Triage

This MR merges the existing partial triage instructions on https://about.gitlab.com/handbook/engineering/quality/issue-triage/#partial-triage with the steps in the /issuetriage TextBlaze template.

It also changes the format from a definition of done into a checklist.

A lot of things are intentionally left out to keep this first iteration small. You can see them at https://gitlab.com/gitlab-org/quality/contributor-success/team-task/-/issues/160. I'll add them in subsequent MRs right after this one ships. A preview of how the final list could look like is at !118187 (closed).

Why is this change being made?

• Reduce redundancy, SSOT, handbook-first
• Make the guide to perform initial issue triage more actionable and human-friendly in the form of a checklist.
• More reasons at https://gitlab.com/gitlab-org/quality/contributor-success/team-task/-/issues/160.

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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