Reorganizing Content

Looking through the guides, I think the nav structure could look like this:

```
Intro
- Ember CLI
CLI Commands
- /index (links to full API docs and references `--help`
- new
- addon
- install
- serve
- build
- generate
- test
Build Pipeline
- Builds & Rebuilds
- Asset compilation
- Dependencies
- Addons (for merging behavior)
- Plugins (Broccoli intro)
- Looking Forward (Ember Auto Import & Embroider)
Addons
- Structure
- Dependencies
- Blueprints
- In-Repo
- Documenting
API Documentation
- CLI
- Addon (aka `index.js` export)
Configuration
- .ember-cli file
- ember-cli-build.js
- ember-cli-update.json
Troubleshooting
- Windows
- Vagrant
- Debugging
- Performance
- Updating
```

Some other things that I can't find a place to put are

- FastBoot: i.e. how to handle FastBoot cases without including FB as a dependency and how to write tests for FastBoot
- Testing (running tests at `localhost:4200/tests`, the fact that the app is mounted in a div, not an iframe, etc)