WIP: Add Think Big information to UX Handbook

Why is this change being made?

• Add SSOT information about Think Big sessions to the UX handbook
• Centralized place where we can reference the process
• Add list of teams/designers and their Think Big efforts

Related to !46575 (comment 324647079)

How will this merge request impact page speed?

No

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what