[scroll-animations] add a "reduced motion" example to the spec

[cookiecrook]

https://drafts.csswg.org/scroll-animations-1/

Since all or most of the examples used in the scroll-animations spec are known vestibular motion triggers, please consider including an example that changes one of the default animations to a dissolve effect (or another animation that is not a vestibular motion trigger) when @media (prefers-reduced-motion) is active.

Please don't just kill all animations in the example when PRM is active. Lots of web devs incorrectly assume that's required, and it often makes the page less usable. There are examples of non-bothersome motion/animation in the linked post. Thanks.