[Docs] Garden path tutorial (bottom-up approach)

[b-admike]

Story

As a LoopBack4 user, I would like an easy to follow and intuitive tutorial which shows me how to make a starter To-Do app like Angular's Tour of Heroes.

This tutorial will be a bottom-up approach tutorial.

See also #735

Acceptance Criteria

• Migrate the step-by-step guide on strongloop/loopback4-example-getting-started to a section called Tutorial for v4 on loopback.io
  • Add an introduction explaining what application the user will be building (Todo)
  • Add explanations at each phase of the tutorial for what the user is currently creating (if it's not already there)
• Hallway-usability-test this on people (devs) and gather feedback
• Blog post

[bajtos]

We already have a Todo application implemented here: https://github.com/strongloop/loopback4-example-getting-started

I agree it makes sense to write a tutorial showing new LB4 developers how easy it is to write such app.