Update the guides to remove usage of `.get`

[locks]

In Ember v3.1.0, you can now omit .get to retrieve computed properties.

Tasks

• [ ] Update code samples to remove .get where that is possible
• [ ] Explain the situations where .get is still required in "The Object Model"
• [ ] Run the codemod in https://github.com/ember-learn/super-rentals and make sure code snippets in the Guides match