Update addons / libraries / dependencies documentation to better represent the current state of npm support

Motivation is so that devs just quick skimming the this page: https://guides.emberjs.com/v3.24.0/addons-and-dependencies/
don't feel like they need to install broccoli transform addons and do `app.import` for every dependency.

This is the only mention that I could find/skim of how things actually work

Based on conversations in Discord lately,
I think the page should be more like:
- explain what can go in the browser and what can't (node vs module/browser and how package.json plays in to that)
- some common things people might want to do with ember-auto-import
- much later down in the page, explain advanced stuff / legacy things (the current stuff)