Add UX Research to Engineering section of TOC.

Why is this change being made?

UX Research is currently not listed in this top level table of contents. I've added it in the interest of increasing findability of our UXR content and illustrating where UXR sits in the overall org structure.

Does this MR meet the acceptance criteria?

Conformity

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