Document mocks and fixtures by node server

In the old site, we had a section about the creation of node servers

**Old site** - https://ember-cli.com/user-guide/#mocks-and-fixtures
**Ember CLI blueprint** - https://github.com/ember-cli/ember-cli/tree/master/blueprints/http-mock/files/server/mocks

We have a good example of the usage here on ESA addon - https://github.com/simplabs/ember-simple-auth/tree/master/packages/test-app/server

cc: @jenweber