Clarify code examples in Getting Started Guide

This PR builds on #14828, #14822 and #14815.

Update the Getting Started tutorial to clarify where and how to add
controller actions.

Previously, beginners could find themselves adding controller actions without
indentation, and, after the example code to add the create and private
article_params methods, they could end up adding the following controller
actions after the private method, which does not work and leaves them confused.
There was also no complete controller code example for them to verify if they
followed the tutorial correctly.

This PR should hopefully help avoid some confusion for them.

[skip ci]