feat(UIKit): Create Figma variables guidelines
@katiemacoy: I guess the main point I was thinking of is for those working in the component library file, we do not want them to add a new variable collection. We do not want them to control properties with variables unless it's to keep integrated components in sync.
If I'm working in a branch and I see Figma variables managing a component's properties, I may think that is our preferred solution. When in reality, we only use these for integrated components.
I am wondering if that can be more spelled out somewhere in this MR - local variables are only to be used for this case.