Adds pair designing guidelines to UX Handbook
Why is this change being made?
- Adds info related to Pair Designing to the UX Handbook
- Related to gitlab-org/gitlab-design#844 (closed)
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?
- Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what