Clarify code examples in Getting Started Guide

Updates the tutorial to clarify where and how to add controller
actions, and in which order as per convention.

Previously, beginners could mistakenly add controller actions
without indentation, and above all, after the example code to
add the `create` and private `article_params` methods, they
could easily add the controller actions that came after underneath
the `private` method, which does not work and leaves them confused.

This PR should hopefully help avoid some confusion for them and
improve their first experience with Rails.

[skip ci]