Update doc examples with ES6 Classes

[Gorzas]

Today I was using ember-infinity and I needed to replicate the example where there's no total_pages and count in the backend's response. The example didn't work because InfinityModel is now an ES6 class but the example is using the EmberObject.extend approach.

As native classes are supported from Ember > 3.4, then there's no problem to change the example with the following code:

import InfinityModel from 'ember-infinity/lib/infinity-model';

class ExtendedInfinityModel extends InfinityModel {
  canLoadMore = true;
}

Sending PR with more clarification. Could somebody confirm this can be done in the rest of examples?