Use case: 404/403 pages

1. Install the Simple Block module which allows you to create simple text blocks that are exportable, these blocks will be used to serve the content for our 404 page.
   1. To create a simple block, go to Administration -> Structure -> Block layout -> Simple blocks
2. Create a new page called 404.
   1. go to Administration -> Structure -> Pages
   2. Set the page path to /404
3. Go to the Layout for that page and add your Simple block.
   1. go to Administration -> Structure -> Pages
   2. Press the Layout button for your newly created 404 page
4. Set the Default 404 under Basic site settings to your newly created page path. 
   1. go to Administration -> Configuration -> Basic site settings
   2. Set the Default 404 under Error pages to be /404

Now visit a URL that does not exists and you will see your 404 page! Following the same instructions you can create a page for 403 as well.