chore(demo): add css-gradient demo

[joshuaellis]

Why

What

Checklist

• Documentation updated
• Demo added
• Ready to be merged

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Latest commit: 1143c5e

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Name	Status	Preview	Updated
react-spring-beta	🔄 Building (Inspect)		Nov 21, 2022 at 7:37PM (UTC)
react-spring-io	🔄 Building (Inspect)		Nov 21, 2022 at 7:37PM (UTC)

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Latest deployment of this branch, based on commit 1143c5e:

Sandbox	Source
Animating Auto	Configuration
Card	Configuration
Cards Stack	Configuration
Chaining Transition and a Spring	Configuration
CSS Gradients	Configuration
CSS Keyframes	Configuration
Async CSS variables	Configuration
Draggable List	Configuration
Exit Before Enter	Configuration
Flip Card	Configuration
Floating Button	Configuration
Goo Blobs	Configuration
Image Fade	Configuration
List Reordering	Configuration
Masonry Grid	Configuration
Multistage Transitions	Configuration
Notification Hub	Configuration
Horizontal Parallax	Configuration
Sticky Parallax	Configuration
Vertical Parallax	Configuration
Rocket decay	Configuration
Basic Transition	Configuration
Native Slide	Configuration
Smile Grid	Configuration
Springy Boxes	Configuration