[packages/doc] define and document package rules

Summary:
Central place for putting package-related info. This document or parts
of it can also be easily ported to other places if deemed appropriate.

This is a backport of core#22674
https://github.com/bitcoin/bitcoin/pull/22674/commits/d59ddc5c3d1c035474d7bc9fa9f8a0eeb1c8498c

Test Plan: proofreading

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12479