Before use routeGenerator, screen should register route first

[BryanYin]

Environment

• Documentation version: 7.2
• Part of the documentation: Manual
• Existing page: https://doc.cuba-platform.com/manual-7.2/url_routes_generator.html

Description of the bug or enhancement

In this section, we need to mention for custom screens, developer should use @Route to register screen route, otherwise, an Exception will throw:

Caused by: java.lang.IllegalStateException: Unable to generate route for screen 'magic_VideoPlayerScreen' - no registered route found
	at com.haulmont.cuba.web.sys.WebUrlRouting$RouteGeneratorImpl.getRoute(WebUrlRouting.java:439) ~[cuba-web-7.2.10.jar:7.2.10]
	at com.haulmont.cuba.web.sys.WebUrlRouting$RouteGeneratorImpl.getRoute(WebUrlRouting.java:483) ~[cuba-web-7.2.10.jar:7.2.10]