Add a Hilla guide with a download request handler example.

Hilla does not support HTTP GET request handlers at the moment, but downloads in Hilla applications could still be implemented using Spring MVC and Spring Security for authentication.

Use case: as a developer, I want to enable application users to download files in a secure way from the server. I want my client-side view to show the server generated download link for the specific file, so that the file download starts when the user clicks the link.

The download response is using the MIME file type and the file name provided by the server.

Live example is not necessary, but let us still put the related Java code to the docs repo.