Update "Managing Dependencies"

[locks]

Goal

Update the Addons and Dependencies/Managing Dependencies sub-section of the Ember Guides. (https://guides.emberjs.com/release/addons-and-dependencies/managing-dependencies/)

To Do

• Expand the Addons section by adding 2-3 paragraphs that explain more about the addon ecosystem, and maybe expound on what ember observer is, and what you can expect to find there.
• Update the explanation for ember-auto-import to the "regular npm packages" section

[Turbo87]

@locks with all this keep in mind that ember-auto-import does not compile dependencies with Babel, so if they are incompatible with your browser targets you're out of luck (see embroider-build/ember-auto-import#41)

[pablobm]

@locks - I believe the first of the two checkboxes can be ticked as per #326

[acorncom]

thanks @pablobm. @locks anything else we want to do here?