Replace repeated list literals in Getting Started guide w/ a constant (rails#49007)

* Replace list literals w/ Visible::VALID_STATUSES

In Getting Started guide, in the article and comment status dropdowns, use Visible::VALID_STATUSES instead of repeating the list of valid statuses.  This makes the code DRYer and more expressive.  This PR also adds a comment to that effect.

* Remove text changes, leave just code

Co-authored-by: Rafael Mendonça França <[email protected]>

Author: davearonson

guides/source/getting_started.md

@@ -1922,7 +1922,7 @@ To finish up, we will add a select box to the forms, and let the user select the
 ```html+erb
 <div>
   <%= form.label :status %><br>
-  <%= form.select :status, ['public', 'private', 'archived'], selected: article.status || 'public' %>
+  <%= form.select :status, Visible::VALID_STATUSES, selected: article.status || 'public' %>
 </div>
 ```
 
@@ -1931,7 +1931,7 @@ and in `app/views/comments/_form.html.erb`:
 ```html+erb
 <p>
   <%= form.label :status %><br>
-  <%= form.select :status, ['public', 'private', 'archived'], selected: 'public' %>
+  <%= form.select :status, Visible::VALID_STATUSES, selected: 'public' %>
 </p>
 ```