How to define a query parameter with REST API?

Yes, from the link you posted there are examples for both sanitising and validating the parameters. Validating does a check which can fail and block the API call from running; sanitising just does some operations to clean or interpret the parameter and does not stop the API call from running.

An example of validation, taken from the page:

<?php
add_action( 'rest_api_init', function () {
  register_rest_route( 'myplugin/v1', '/author/(?P<id>\d+)', array(
    'methods' => 'GET',
    'callback' => 'my_awesome_func',
    'args' => array(
      'id' => array(
        'validate_callback' => function($param, $request, $key) {
          return is_numeric( $param );
        }
      ),
    ),
  ) );
} );

You can see in the second parameter of register_rest_route that this defines an endpoints like /author/1234

The format of the second parameter of register_rest_route breaks down as:

• /author/ initial part of the URL to match
• ?P a code specific to this function that means 'parameter'. Note this is not included in the URL when called
• <id> optional name for the parameter, used belows in args, not included as part of URL.
• \d+ the regex for this parameter