UX: Include subcategories in crawler view (#21227)

Adds a bit more information to the categories view for crawlers, for better indexing of deep content.

This only works when the "Subcategories with Featured Topics" is the selected layout.

app/views/categories/index.html.erb

@@ -20,6 +20,13 @@
                 </a>
               </h3>
               <div itemprop='description'><%= c.description&.html_safe %></div>
+              <% if c.subcategory_list.present? %>
+                <div class='subcategories'>
+                  <% c.subcategory_list.each_with_index do |sc, index| %>
+                    <a href='<%= sc.url %>'><%= sc.name %></a>&nbsp;
+                  <% end %>
+                </div>
+              <% end %>
             </div>
           </td>
           <td class='topics'>