Clarify code examples in Getting Started Guide

Update the tutorial to clarify how and where to add the
`new`, `show` and `index` controller actions.

Beginners mistakenly add the `new` action
without indentation. They then add the `show` and `index`
actions below the previously-shown code adding the `create`
and private `article_params` methods… so after `private`,
which does not work and leaves them confused.

This PR should hopefully help avoid some confusion for them.

[ci skip]