Docs: Make it clear what is allowed/disallowed around copying other designs

This is just a note to myself to write some policy up for this.

The issue names no files, tests, or entry points. Start by locating the repository's existing documentation and any guidance on copying or design attribution, then identify the maintainers or stakeholders who must define the policy. Done means an agreed, published policy clearly distinguishing allowed and disallowed copying.

Written by the indexing model from the issue text.