Hooks and routes not working at all
Creating the files is not enough: The module needs also to be installed. /admin/modules gives a list of modules and allows to install them. 

After a module is installed, all its hooks are invoked from Drupal. 

As side note, YAML files are required to use the correct indentation. The content of the  hello_world.routing.yml file should be the following one.

hello_world.hello:
  path: '/hello'
  defaults:
    _controller: '\Drupal\hello_world\Controller\HelloWorldController::helloWorld'
    _title: 'Our first route'
  requirements:
    _permission: 'access content'


See the example give in Routing API, Registering simple routes.