Improve animation/motion design documentation in terms of easing curves

Problem

https://design.gitlab.com/foundations/motion is currently lacking in terms of documentation and guidance for choosing the correct easing curves for animations.

Proposal

https://vimeo.com/282452432 at ± 26:00 states that using custom easing css curves can have a big impact on the user experience when used well.

Additional sources

Improve documentation with information from:

• https://www.nngroup.com/articles/animation-usability/
• https://www.nngroup.com/articles/progress-indicators/
• https://spectrum.adobe.com/page/motion/