Add Group Opportunity Assessment Template to handbook

Why is this change being made?

Resolves Product#2306 (closed)

Opportunity assessment should be an ongoing activity. In order to facilitate ongoing opportunity review, the medium for capturing the opportunity should be very lightweight.

Furthermore, opportunity assessment is most useful when the decision-maker understands what they are weighing their investment against. Capturing opportunities in a single location can result in more thoughtful decisions on what to invest in.

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.
  • 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 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)