message: Update React router and add Next.js docs
diff: @@ -104,20 +104,61 @@ render(<Navigation />)
  ### With React Router
  ```jsx
  -import {Link} from 'react-router-dom'
  +import {Link, useMatch, useResolvedPath} from 'react-router-dom'
  import {UnderlineNav} from '@primer/react/drafts'
  +
  +function UnderlineNavItem({to, children, ...rest}) {
  + const resolved = useResolvedPath(to)
  + const isCurrent = useMatch({path: resolved.pathname, end: true})
  + return (
  + <UnderlineNav.Item as={Link} to={to} aria-current={isCurrent ? 'page' : undefined} {...rest}>
  + {children}
  + </UnderlineNav.Item>
  + )
  +}
  +
  const Navigation = () => {
  return (
  <UnderlineNav aria-label="Repository">
  - <UnderlineNav.Item as={Link} to="code" counter={4} aria-current="page">
  + <UnderlineNavItem to="/code" counter={4}>
  Code
  - </UnderlineNav.Item>
  - <UnderlineNav.Item counter={44} as={Link} to="issues">
  + </UnderlineNavItem>
  + <UnderlineNavItem to="/issues" counter={44}>
  Issues
  + </UnderlineNavItem>
  + <UnderlineNavItem to="/pulls">Pull Requests</UnderlineNavItem>
  + </UnderlineNav>
  + )
  +}
  +```
  +
  +### With Next.js
  +
  +```jsx
  +import {useRouter} from 'next/router'
  +import Link from 'next/link'
  +import {UnderlineNav} from '@primer/react/drafts'
  +
  +function UnderlineNavItem({href, children, ...rest}) {
  + const router = useRouter()
  + const isCurrent = typeof href === 'string' ? router.asPath === href : router.pathname === href.pathname
  + return (
  + <UnderlineNav.Item s={Link} href={href} aria-current={isCurrent ? 'page' : undefined} {...rest}>
  + {children}
  </UnderlineNav.Item>
  - <UnderlineNav.Item as={Link} to="pulls">
  - Pull Requests
  - </UnderlineNav.Item>
  + )
  +}
  +
  +const Navigation = () => {
  + return (
  + <UnderlineNav aria-label="Repository">
  + <UnderlineNavItem href="/code" counter={4}>
  + Code
  + </UnderlineNavItem>
  + <UnderlineNavItem href="/issues" counter={44}>
  + Issues
  + </UnderlineNavItem>
  + <UnderlineNavItem href="/pulls">Pull Requests</UnderlineNavItem>
  </UnderlineNav>
  )
  }
  