Update process for Scrum Hybrid Milestone Planning

Why is this change being made?

This MR re-implements a scrum process b21a194a for the Product Intelligence team.

Over the past year, our team has been using a Kanban process in order to quickly adjust to changing requirements. Now that requirements are becoming more solidified and with the change in managers, I’d like our team to shift to a Milestone process. I believe a Milestone process will ensure more predictability and will make it easier for incoming managers to manage.

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 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 in on the page being edited.
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  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)