doc: add policy for distribution

This is my attempt at writing down what seems to be a rough consensus emerging in the various Corepack-related threads such as #50963. The purpose of this PR is to confirm that what’s written here is a consensus or majority opinion (if it comes to a vote) to help focus the decision-making around Corepack. This file can always change (or be removed) in the future; all it takes is another PR.

@nodejs/tsc @nodejs/corepack @nodejs/package-maintenance