Vaadin for TypeScript: Document the vaadin-router-location-changed event

[haijian-vaadin]

As a developer, I want to listen to the navigation event so that I can, for example, update the header/ footer of the page.

Vaadin router already fires the vaadin-router-location-changed event which can be used for this purpose, but it's currently not documented anywhere yet.

DoD vaadin-router-location-changed event is included in the router navigation events section A code example of how to use that should also be included.