Ember Data is a secret dependency breaking consumers of ec-addon-docs

It's marked as a `peerDependency`, but it is required for ec-addon-docs to run properly. If the consuming project does not have `ember-data` in its package.json, ec-addon-docs breaks and throws an error, and the docs will not render. However, we can't specify it as a dependency within ec-addon-docs, either.

Addons do not ship with ember-data by default, as it is not part of the addon blueprint. Most addons don't have a need to install it either, as most addons won't be doing any data fetching. So, we cannot count on ember-data to be present in order to ship a working experience to users of this project.

There are three possible solutions which could be used in conjunction with one another:

1) Update the documentation for ec-addon-docs to explicitly ask users to install ember-data as part of the setup guide.
2) Add ember-data to the blueprint so that installing ec-addon-docs ensures ember-data is also installed, when using `ember install`
3) Use version checker to have addon-docs throw an error if a valid version of ember-data (currently 3.0+) is not present in the consuming addon.