[website/doc] Unify splash screen discoverability

It is currently hard to get back to the page where you could download all splash screen scene, when i type on google i only found this page docs.blender.org/manual/en/latest/interface/window_system/splash.html or this one docs.blender.org/manual/en/2.80/interface/splash.html but none redirect to blender.org/download/demo-files/ (and they also don't redirect one blender version to another flawlessly).