Created by: vdepizzol
This PR adds a dedicated "Responsive design" page to the Foundations section, covering Primer's definition of responsive design as per our ADR (private link only available for hubbers).
This is a follow-up to https://github.com/primer/design/pull/305, which already considered other related foundation pages such as this one.
I'm treating this as a living doc. I left a few comments regarding areas we might continue to improve, including more specific touch strategies that are being designed in this Figma file (internal for hubbers).