Add CONTRIBUTING-Figma.md doc
Problem
Decisions for working within Figma are being made in separate issues under the Figma - Documentation epic, but are not being captured anywhere else for consumption.
Solution
Document decisions and workflows for contributing to our Figma Globals library in a new, Figma-specific contributing document. For the time being, ensure that this the document communicates that this is in a pilot/beta status.
/cc @pedroms @tauriedavis