docs: add design topic on copy keyword argument behavior

[kgryte]

This PR

• codifies the discussions from the 23 January 2025 workgroup meeting.
• supersedes docs: clarify copy kwarg behavior in asarray for libraries disallowing mutation #886, closes Disambiguate what copy=True means for dask #866, and closes RFC: add copy #495 by clarifying that array libraries are recommended to always perform a physical copy unless they can guarantee that not doing so allows a returned array to be mutated without side-effects.
• performs some minor copy edits to the existing copy.

[ev-br]

Looks good to me.

[kgryte]

Thanks, @ev-br!