Finalize contribution policy

[knoepfel]

The coding guidelines were established for the F1 milestone, and they currently reside at https://github.com/Framework-R-D/phlex-coding-guidelines. There are additional aspects of a contribution policy that need to be specified. The current draft resides at:

• https://github.com/Framework-R-D/phlex-design/blob/main/external-contributions-policy.md

This should be finalized before January 1, 2026.