Clarify code examples in Getting Started Guide

This PR builds on #14822 and #14815.

Compared to #14822, as per suggestions by @schneems 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.

This PR 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 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]