Clarify code examples in Getting Started Guide

This PR builds on #14822 and #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.

Compared to #14822, it no longer uses ellipsis marks without
a comment hash and adds a complete controller code example
for reference after the destroy action in the tutorial.

Previously, beginners could easily find themselves adding
controller actions without indentation, and, after the example
code to add the create and private article_params methods,
they could easily end up adding controller actions 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]