WIP: Added iteration checklist

Why is this change being made?

Iteration is one of our core values, but we don't have many examples or a framework of how to improve at it. There's plenty of guidance and examples of our other values. There's probably more we can do to help PMs think iteratively and to check themselves on whether or not there's an opportunity to break an issue down further or to change course on a proposal.

This MR attempts to add a series of questions a PM can walk through if they're looking for inspiration. I personally had a hard time thinking of an elegant framework that worked well; when it comes to iteration, I instead think about a pre-flight checklist that I think about if I'm ever in doubt that we've arrived at an MVC.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the change relates to any part of the project other than updates to content and/or data files (e.g. team.yml) please make sure to ping @gl-static-site-editor in a comment for a review and merge.
  • If the DRI for the page/s being updated isn't immediate clear, then assign it to your manager.
  • 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

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