Resource Handler documentation

[Cleptomania]

Arcade has a resource handler system, which is what the built-in :resources: system uses. This system allows users to add their own custom resource handles that function the same way, however this is not currently documented anywhere.

The system is fairly simple, you just use arcade.resources.add_resource_handler("my_handle", "my_filepath") and it resolves the filepath and adds it to the dictionary for resource handles.

This could maybe be added to the built-in resources page, or perhaps should have it's own page under the API docs?