Add CSS Animations Library entry

[sonnyp]

The only occurrence we have is on the Welcome entry but it's just for show for the first time introduction.

• The CSS Animations entry should have an animated gradient and mention the CSS Gradients demo library: Add CSS gradients demo sonnyp/Workbench#636
• We can animate Image scale like on the welcome
• What else?

There is also sonnyp/Workbench#647 with an animated CSS gradient example