[PP-1] Document when and how PackageManagerFixtureCreator should be used

Problem/Motivation

This is spun off from ✨ Add Alpha level Experimental Package Manager module Needs review , specifically this MR comment.

Package Manager includes a developer-facing utility class called PackageManagerFixtureCreator but it's not clear how, when, or even if it should be used. Let's get clarity on that here, and add the requisite documentation. If it turns out to be not needed at all, we can remove it from the codebase instead.