Draft: Create checklist for issue triage

Why is this change being made?

We currently have several places that define issue triage:

• the partial triage section
• the triaging issues section
• the /issuetriage TextBlaze snippet
• other parts of the issue triage page contain plenty of additional things to keep in mind while triaging issues

It would be nice to have a SSOT checklist for triaging issues that every issue triager could go through. A checklist helps reduce failure in repetitive tasks by compensating for potential limits of human memory and attention. It helps to ensure consistency and completeness in carrying out a task.

This MR adds a detailed checklist for each triage type. It also re-organizes the affected sections on the issue triage page around this new triage checklist. No changes to policies. This MR merely moves and merges already existing information.

Since there are a lot of changes, I recommend a two-step review process. The first review round focus only on the checklists. The goal for the first review round is reaching agreement about what exactly happens during each triage type.

1. Does the "partial triage" checklist make sense?
2. Does the "complete triage" checklist make sense?
3. Is there a "secondary triage"?

Once we have agreement on the triage types and what exactly happens in each, I'll adjust the other changes in this MR accordingly for the second review round.

@gl-quality/contributor-success

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
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  • 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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