Add documentation for drag and drop in React Aria (#3423)

* Focus nearest drop target

* Fix bugs in useDrop

* Expose more states from hooks

* Make drop target hit testing reusable

* Start of aria dnd docs

* Update tests

* Focus nearest drop target

* Fix bugs in useDrop

* Expose more states from hooks

* Make drop target hit testing reusable

# Conflicts:
#	packages/@react-aria/dnd/src/index.ts

* Update tests

* fix merge

* Fix ListLayout dnd for persisted keys

* Update packages/@react-stately/virtualizer/src/Virtualizer.ts

Co-authored-by: Robert Snow <[email protected]>

* Workaround bugs in WebKit and Chrome

* Highlight important sections of example code

* sprinkle css throughout

* Support onAction in useListBox

* Support useDrag without an explicit drag affordance

* Add useDroppableCollection docs

* Add more sections to useDroppableCollection docs

* Add more introduction

* Add dnd introduction page

* Copy updates

* Update docs for new API

* Fix auto scrolling in examples in safari

* Update last example

* Fix directory examples

* review updates

* Update color scheme for dark mode contrast

* Remove onDropActivate

* Add missing prop and function descriptions

* Make highlights line up with other code

Co-authored-by: Robert Snow <[email protected]>
Co-authored-by: Danni <[email protected]>

Author: 3 people