@PixelGeek did a workshop about it. Read here : Webflow Workshop #114: Creating a page transition effect and check the links down in the comments.
Also this blog post by @Aaron: https://webflow.com/blog/how-to-build-page-transitions-in-webflow
@Blaise_Posmyouck did a great job: https://www.blaiseposmyouck.com/