Show the ember-cli logo above the TOC

For the cli-guides we should show the ember-cli logo in the top left just above the TOC.

As we are using guidemaker for the ember-cli guides we will need to make this a configurable feature of the [guidemaker-ember-template](https://github.com/ember-learn/guidemaker-ember-template) so that any other projects using the same template can optionally configure this logo

### Todo

- [ ] add a configuration entry for `logo` to the guidemaker block of the cli-guides https://github.com/ember-learn/cli-guides/blob/master/config/environment.js#L33
- I guess it doesn't really need to be in the guidemaker block 🤔 but it needs to be somewhere in that config
- [ ] if the logo is configured it should be used in the template above the TOC
- it might be worth putting it in this file that renders the left column: https://github.com/ember-learn/guidemaker-ember-template/blob/master/app/templates/version.hbs