Clarify code examples in Getting Started Guide

Clarify how and where to add the `new`, `show` and `index`
controller actions in the Getting Started tutorial.

From observation, beginners mistakenly add the `new` action
without indentation. They then add the `show` and `index`
actions without indentation and underneath the previous code
example for the `create` and private `article_params` methods…
therefore after `private`, which does not work and leaves them
confused.

This PR may hopefully help avoid some confusion for them.

[ci skip]