docs(gatsby): Add documentation for useStaticQuery (#11741)

docs/docs/use-static-query.md

@@ -0,0 +1,51 @@
+---
+title: Querying data in components with the useStaticQuery hook
+---
+
+Gatsby v2.1.0 introduces `useStaticQuery`, a new Gatsby feature that provides the ability to use a [React Hook](https://reactjs.org/docs/hooks-intro.html) to query with GraphQL at _build time_.
+
+Just like the [StaticQuery](/docs/static-query/) component, it allows your React components to retrieve data via a GraphQL query that will be parsed, evaluated, and injected into the component. However, `useStaticQuery` is a hook rather than a component that takes a render prop!
+
+In this guide, we'll walk through an example using `useStaticQuery`. If you're not familiar with static queries in Gatsby, you might want to check out [the difference between a static query and a page query](/docs/static-query/#how-staticquery-differs-from-page-query).
+
+## How to use useStaticQuery in components
+
+> 💡 You'll need React and ReactDOM 16.8.0 or later to use `useStaticQuery`.
+>
+> 📦 `npm install react@^16.8.0 react-dom@^16.8.0`
+
+`useStaticQuery` is a React Hook. All the [Rules of Hooks](https://reactjs.org/docs/hooks-rules.html) apply.
+
+It takes your GraphQL query and returns the requested data. That's it!
+
+### Basic example
+
+Let's create a `Header` component that queries for the site title from `gatsby-config.js`:
+
+```jsx:title=src/components/header.js
+import React from "react"
+import { useStaticQuery, graphql } from "gatsby"
+
+export default () => {
+  const data = useStaticQuery(graphql`
+    query HeaderQuery {
+      site {
+        siteMetadata {
+          title
+        }
+      }
+    }
+  `)
+
+  return (
+    <header>
+      <h1>{data.site.siteMetadata.title}</h1>
+    </header>
+  )
+}
+```
+
+## Known Limitations
+
+- `useStaticQuery` does not accept variables (hence the name "static"), but can be used in _any_ component, including pages
+- Because of how queries currently work in Gatsby, we support only a single instance of `useStaticQuery` in a file

www/src/data/sidebars/doc-links.yaml

@@ -118,6 +118,8 @@
           link: /docs/page-query/
         - title: Querying data in components with StaticQuery
           link: /docs/static-query/
+        - title: Querying data in components with the useStaticQuery hook
+          link: /docs/use-static-query/
         - title: Using GraphQL fragments
           link: /docs/using-fragments/
         - title: Creating slugs for pages