Summary:
After taking a look at the existing animation docs, I found that most of the documentation on using `Animated` was spread out throughout the Animations guide and the `Animated` API reference, without any particular structure in place.
This PR aims to clean up the API reference, focusing on documenting all the provided methods exhaustively, and deferring to the Animations guide for long form examples and supporting content.
The `Easing` module is referred to at various points in the API reference, so I decided to clean up this doc as well. easings.net provides some handy visualizations that should make it easier for the reader to understand what sort of easing curve each method provides.
The site was built locally, and I verified all three documents render correctly.
Closes https://github.com/facebook/react-native/pull/12410
Differential Revision: D4581314
Pulled By: hramos
fbshipit-source-id: 27c0bce2afac8f084311b9d6113a2641133b42e5
6ad41a81bc
