Clarify explanations in Getting Started Guide

This PR builds on and completes #14815.

It updates the getting started tutorial to clarify where and how to
add controller actions, and in which order as per convention.

It also attempts to improve some of the grammar and explanations.

Previously, beginners could mistakenly add controller actions
without indentation, and, after the example code to add the `create`
and private `article_params` methods, they could easily be led to
add the controller actions that came afterward 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]