Update product designer onboarding to mention the GitLab Figma plugin

Emily Sybrantrequested to merge
esybrant-figma-plugin into master

Adds information about the GitLab Figma plugin to the product designer onboarding issue template. The GitLab Figma plugin is a much easier way to add designs to issues than exporting designs as PNGs and uploading them, and it's valuable for product designers to know about this workflow.

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