Add example using `default_main_content` to show-pages.md (#6487)
Add example using `default_main_content` to show-pages.md (#6487)

See #6486
javierjulio/activeadmin,javierjulio/activeadmin,activeadmin/activeadmin,deivid-rodriguez/activeadmin,javierjulio/activeadmin,deivid-rodriguez/activeadmin,activeadmin/activeadmin,deivid-rodriguez/activeadmin,activeadmin/activeadmin