Google for a new utility called "Drupal Console". This is a fantastic console script
that helps you debug, clear cache and generate code. If you love it, you should say
thank you to Jesus Olivas and others for their work!
It's a bit like Drush, but different, but kind of the same... I don't know. They
seem to be co-existing and don't do the exact same things.
To download it, copy the curl statement and run that in your terminal. Next, move
it into a global bin directory so that you can simply type drupal from anywhere
in the terminal:
One of those commands is router:rebuild. Run that to clear the routing cache:
Ok, go back, refresh and congratulations!!! Seriously: you've just created your first
custom page in Drupal 8. By the way, creating a page in the Symfony framework looks
almost exactly the same. You're mastering two tools at once! You deserve a vacation.
Notice this page is literally only the text "ROOOOAR". It doesn't have any theming
or templates applied to it. We will tap into the theme system later, but this is
really interesting: if you want to return a Response all by yourself, you can
do that and Drupal won't mess with it. Hey, this could even be a JSON response for
an API. Drupal is a CMS, but it's also a modern, custom-development framework.