Add ThinkBig instructions to the UX Handbook

Why is this change being made?

Several teams have started utilizing a special sync session called "ThinkBIG!" with their teams to collaborate and harmonize on stage/group level strategy. This MR adds the overview, instructions, and suggested modules to the Handbook so new teams can utilize the same methodology.

NOTE: This MR replaces !46646 (closed) to resolve a technical issue with accidentally deleted branches.

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 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.
  • 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.

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