Use a standalone app for docs

[simonihmig]

The current setup of using the addon's dummy app for the docs site has caused conflicts more than once, e.g. using latest features for the docs would break ember-try scenarios of older Ember versions for the addon. Also dependencies of the docs can cause other unwanted side-effects in the addon tests.

Having the docs as a separate app with its separate dependencies and build seems better overall!?

Related: ember-learn/ember-cli-addon-docs#409