Figma > Depricate Sketch contributor guidelines

We've created https://gitlab.com/gitlab-org/gitlab-design/-/blob/master/CONTRIBUTING-Figma.md to keep track of how to contribute to Figma. When we switch over, we'll want to depricate the guidelines related to Sketch in https://gitlab.com/gitlab-org/gitlab-design/-/blob/master/CONTRIBUTING.md and move CONTRIBUTING-Figma.md to CONTRIBUTING.md