Document new and improved helper functions experience

[jenweber]

Excellent news - helpers are now much more intuitive to use in Ember! The feature shipped quietly, and now that it's had some additional testing, we should document it and invite all our developers to start using it!

This issue blocks advancing this RFC: https://github.com/emberjs/rfcs/pull/866

How to help:

• Read the RFC, https://rfcs.emberjs.com/id/0756-helper-default-manager
  • Especially review the "How we teach this" section of the RFC
• Update the page on helpers as described in the RFC. When possible, re-use examples from the RFC.
• Find all the additional places where helper is mentioned in the guides/release folder of this project
• Update those sections too, or open new tickets for them if they are not quick to fix

Excerpt from "how we teach this":

On the Helper Functions page, We'll want to insert a section early on about how helpers can be "local" to or defined on components and controllers. Then, once there have been examples of the local/private helper, the existing content can continue to talk about the "global helper" -- explicitly differentiating between the local/private helper, rather then retaining the general "Helper Function" title as "Helper Functions" are all of these, rather than just the globally defines ones.

[SkoebaSteve]

As discussed I'll take this and will create a PR by next week, thanks for the introduction @jenweber!