Add design system rules for GitLab Duo Code Review

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Add items to .gitlab/duo/mr-review-instructions.yaml that help Duo give better reviews about use of the design system.

At the time of writing, Duo Code Review cannot follow arbitrary links, so we cannot link to https://design.gitlab.com, but I think we can link to files within the repository itself (see Enable Duo Code Review to read repo files in cu... (#580337 - closed)).