Octane - new content to help avoid jQuery confusion

[jenweber]

Purpose

This PR helps address emberjs/rfc-tracking#3

The goal is to reflect that Octane will not have jQuery by default, and to give specific pointers to people about how to make API requests.

Added

• A new top-level topic and Guide that lays out options for loading data in an Ember app 🔥. This is a continuation of the effort to untangle how we teach Ember Data. This section will have more articles as time goes on and we get to the point where Ember Data can be a stand-alone guide.

Changed

• I modified the "optional features" guide to reflect my understanding of the way jQuery will work in Octane apps
• I removed a lingering jQuery reference

Departures from RFC 431

I renamed "data management" to be "Making back end requests" because the "state management" topic clashes with the original name. I reviewed state management and I think it's too confusing to try and add this data fetching stuff to that guide. I expect it to keep growing.

I am open to renaming this section.

Next steps:

• another PR that finds the places where we should link to these pages
• In places where we mention a fetch polyfill, instead move that info to this page and just link to this resource

[pzuraq]

Generally looks good! I think the "Back End Requests" name is a bit of a colloquialism, most people should get it and it may be better than the alternatives: "Making Network Requests", "Making API Requests", etc. I'm 👍 either way

[jenweber]

Thanks for the feedback @jelhan and @pzuraq! I updated this PR to include it.