Add new docs website (#10896)

Adds a new docs website, built with Gatsby JS, to replace the old Jekyll site. Source code for the new site lives in /www (although markdown and YML data still comes from the legacy /docs folder).

Changes to either markdown or website source code can be previewed on Netlify. The react-js bot should automatically add comments to each PR with preview links. (This preview is generated by running the newly-added yarn build:docs command in the root package.json.)

The majority of the changes in this PR are contained within the new /www directory. However some minor modifications have been made to existing content in the /docs directory:

* Modified frontmatter author block to always be an array
* Small markdown formatting tweaks

docs/reference-react.md

@@ -25,16 +25,16 @@ React components let you split the UI into independent, reusable pieces, and thi
  - [`React.Component`](#react.component)
  - [`React.PureComponent`](#react.purecomponent)
 
-If you don't use ES6 classes, you may use the `create-react-class` module instead. See [Using React without ES6](/react/docs/react-without-es6.html) for more information.
+If you don't use ES6 classes, you may use the `create-react-class` module instead. See [Using React without ES6](/docs/react-without-es6.html) for more information.
 
 ### Creating React Elements
 
-We recommend [using JSX](/react/docs/introducing-jsx.html) to describe what your UI should look like. Each JSX element is just syntactic sugar for calling [`React.createElement()`](#createelement). You will not typically invoke the following methods directly if you are using JSX.
+We recommend [using JSX](/docs/introducing-jsx.html) to describe what your UI should look like. Each JSX element is just syntactic sugar for calling [`React.createElement()`](#createelement). You will not typically invoke the following methods directly if you are using JSX.
 
 - [`createElement()`](#createelement)
 - [`createFactory()`](#createfactory)
 
-See [Using React without JSX](/react/docs/react-without-jsx.html) for more information.
+See [Using React without JSX](/docs/react-without-jsx.html) for more information.
 
 ### Transforming Elements
 
@@ -60,19 +60,19 @@ class Greeting extends React.Component {
 }
 ```
 
-See the [React.Component API Reference](/react/docs/react-component.html) for a list of methods and properties related to the base `React.Component` class.
+See the [React.Component API Reference](/docs/react-component.html) for a list of methods and properties related to the base `React.Component` class.
 
 * * *
 
 ### `React.PureComponent`
 
-`React.PureComponent` is exactly like [`React.Component`](#react.component) but implements [`shouldComponentUpdate()`](/react/docs/react-component.html#shouldcomponentupdate) with a shallow prop and state comparison.
+`React.PureComponent` is exactly like [`React.Component`](#react.component) but implements [`shouldComponentUpdate()`](/docs/react-component.html#shouldcomponentupdate) with a shallow prop and state comparison.
 
 If your React component's `render()` function renders the same result given the same props and state, you can use `React.PureComponent` for a performance boost in some cases.
 
 > Note
 
-> `React.PureComponent`'s `shouldComponentUpdate()` only shallowly compares the objects. If these contain complex data structures, it may produce false-negatives for deeper differences. Only extend `PureComponent` when you expect to have simple props and state, or use [`forceUpdate()`](/react/docs/react-component.html#forceupdate) when you know deep data structures have changed. Or, consider using [immutable objects](https://facebook.github.io/immutable-js/) to facilitate fast comparisons of nested data.
+> `React.PureComponent`'s `shouldComponentUpdate()` only shallowly compares the objects. If these contain complex data structures, it may produce false-negatives for deeper differences. Only extend `PureComponent` when you expect to have simple props and state, or use [`forceUpdate()`](/docs/react-component.html#forceupdate) when you know deep data structures have changed. Or, consider using [immutable objects](https://facebook.github.io/immutable-js/) to facilitate fast comparisons of nested data.
 >
 > Furthermore, `React.PureComponent`'s `shouldComponentUpdate()` skips prop updates for the whole component subtree. Make sure all the children components are also "pure".
 
@@ -88,11 +88,11 @@ React.createElement(
 )
 ```
 
-Create and return a new [React element](/react/docs/rendering-elements.html) of the given type. The type argument can be either a tag name string (such as `'div'` or `'span'`), or a [React component](/react/docs/components-and-props.html) type (a class or a function).
+Create and return a new [React element](/docs/rendering-elements.html) of the given type. The type argument can be either a tag name string (such as `'div'` or `'span'`), or a [React component](/docs/components-and-props.html) type (a class or a function).
 
 Convenience wrappers around `React.createElement()` for DOM components are provided by `React.DOM`. For example, `React.DOM.a(...)` is a convenience wrapper for `React.createElement('a', ...)`. They are considered legacy, and we encourage you to either use JSX or use `React.createElement()` directly instead.
 
-Code written with [JSX](/react/docs/introducing-jsx.html) will be converted to use `React.createElement()`. You will not typically invoke `React.createElement()` directly if you are using JSX. See [React Without JSX](/react/docs/react-without-jsx.html) to learn more.
+Code written with [JSX](/docs/introducing-jsx.html) will be converted to use `React.createElement()`. You will not typically invoke `React.createElement()` directly if you are using JSX. See [React Without JSX](/docs/react-without-jsx.html) to learn more.
 
 * * *
 
@@ -126,11 +126,11 @@ This API was introduced as a replacement of the deprecated `React.addons.cloneWi
 React.createFactory(type)
 ```
 
-Return a function that produces React elements of a given type. Like [`React.createElement()`](#createElement), the type argument can be either a tag name string (such as `'div'` or `'span'`), or a [React component](/react/docs/components-and-props.html) type (a class or a function).
+Return a function that produces React elements of a given type. Like [`React.createElement()`](#createElement), the type argument can be either a tag name string (such as `'div'` or `'span'`), or a [React component](/docs/components-and-props.html) type (a class or a function).
 
 This helper is considered legacy, and we encourage you to either use JSX or use `React.createElement()` directly instead.
 
-You will not typically invoke `React.createFactory()` directly if you are using JSX. See [React Without JSX](/react/docs/react-without-jsx.html) to learn more.
+You will not typically invoke `React.createFactory()` directly if you are using JSX. See [React Without JSX](/docs/react-without-jsx.html) to learn more.
 
 * * *