[landing] Swap out cloud animation lottie asset and adjust scroll config

Summary: Replace simple cloud animation with the more complex one. Adjust scroll config so animation only starts once in full view.

Test Plan: Looks as expected on Safari/Chrome/Firefox

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, palys-swm, Adrian

Differential Revision: https://phabricator.ashoat.com/D1095