updating UI documentation

Summary:
adding documentation for more of our UI components.

Deleted some unused components, which were not working anyways.

Reviewed By: jknoxville

Differential Revision: D12896109

fbshipit-source-id: 959c7864240883869ad67283f80a3c189b94bf00

src/ui/components/FlexCenter.js

@@ -8,6 +8,9 @@
 import View from './View.js';
 import styled from '../styled/index.js';
 
+/**
+ * A container dispalying its children horizontally and vertically centered.
+ */
 export default styled(View)({
   display: 'flex',
   alignItems: 'center',