WIP - Create a dedicated section on application data flow/requests

This topic is still under discussion and isn't ready yet to be picked up by contributors. The goal is to first work out some of the big questions of what this section should look like, acceptance criteria, MVP, etc.

The proposal is to create a new topic under Core Concepts where we tackle questions like "where should I load data," loading data in components, etc. It is motivated by the popularity of this Readers' Question, and that we ought to have something octaney here: https://discuss.emberjs.com/t/readers-questions-is-it-bad-to-load-data-in-components/14521