Convert the angular-green-grocers exemplar into an exercise

[Auenc]

The angular-green-grocers is an exemplar for the third exercise in Angular. The idea behind the exercise is to introduce state and Observables

What needs to be done:

• [x] Create the learning objectives for the exercise
• [x] Add instructions for setting up the exercise
• [x] Come up with requirements that will bring the student to the finished exercise.

[Auenc]

Here is the link to the exercise version: https://github.com/boolean-uk/angular-greengrocers

[Auenc]

Some changes needed to use the api endpoint, and update the exemplar to reflect this. Additionally, Observables need to be switched to promises