Add "Migrating from React Router" docs. (#20982)

Please let me know if anything is missing from this document 😄

docs/migrating/from-react-router.md

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+---
+description: Learn how to migrate from React Router to file-system based routes with Next.js.
+---
+
+# Migrating from React Router
+
+This guide will help you understand how to transition from [React Router](https://reactrouter.com) to [file-system based](/docs/routing/introduction.md) routes with Next.js. Using [`next/link`](/docs/api-reference/next/link.md) and [`next/router`](/docs/api-reference/next/router.md) will allow you to:
+
+- Decrease bundle size by removing React Router as a dependency.
+- Define your application routes through the file system.
+- Utilize the latest improvements to the Next.js framework.
+
+## Basics
+
+First, uninstall React Router. You'll be migrating to the built-in routing with Next.js.
+
+```jsx
+npm uninstall react-router-dom
+```
+
+The `Link` component for performing client-side route transitions is slightly different from React Router.
+
+```jsx
+// Before (React Router)
+import { Link } from 'react-router-dom'
+
+export default function App() {
+  return <Link to="/about">About</Link>
+}
+
+// After (Next.js)
+import Link from 'next/link'
+
+export default function App() {
+  return (
+    <Link href="/about">
+      <a>About</a>
+    </Link>
+  )
+}
+```
+
+Most React applications that use React Router have a top-level navigation file, containing a list of routes. For example:
+
+```jsx
+import { BrowserRouter as Router, Switch, Route } from 'react-router-dom'
+
+export default function App() {
+  return (
+    <Router>
+      <Switch>
+        <Route path="/about">
+          <h1>About</h1>
+        </Route>
+        <Route path="/blog">
+          <h1>Blog</h1>
+        </Route>
+        <Route path="/">
+          <h1>Home</h1>
+        </Route>
+      </Switch>
+    </Router>
+  )
+}
+```
+
+With Next.js, you can express the same application structure in the file system. When a file is added to the [`pages`](/docs/basic-features/pages.md) directory it's automatically available as a route.
+
+- `pages/about.js` → `/about`
+- `pages/blog.js` → `/blog`
+- `pages/index.js` → `/`
+
+## Nested Routes
+
+In the example below, routes like `/blog/my-post` would render the `Post` component. If a slug was not provided, it would render the list of all blog posts.
+
+```jsx
+import {
+  BrowserRouter as Router,
+  Switch,
+  Route,
+  useRouteMatch,
+  useParams,
+} from 'react-router-dom'
+
+export default function Blog() {
+  // Nested route under /blog
+  const match = useRouteMatch()
+
+  return (
+    <Router>
+      <Switch>
+        <Route path={`${match.path}/:slug`}>
+          <Post />
+        </Route>
+        <Route path={match.path}>
+          <h1>All Blog Posts</h1>
+        </Route>
+      </Switch>
+    </Router>
+  )
+}
+
+function Post() {
+  const { slug } = useParams()
+  return <h1>Post Slug: {slug}</h1>
+}
+```
+
+Rather than using the `:slug` syntax inside your `Route` component, Next.js uses the `[slug]` syntax in the file name for [Dynamic Routes](/docs/routing/dynamic-routes.md). We can transform this to Next.js by creating two new files, `pages/blog/index.js` (showing all pages) and `pages/blog/[slug].js` (showing an individual post).
+
+```jsx
+// pages/blog/index.js
+
+export default function Blog() {
+  return <h1>All Blog Posts</h1>
+}
+
+// pages/blog/[slug].js
+
+import { useRouter } from 'next/router'
+
+export default function Post() {
+  const router = useRouter()
+  const { slug } = router.query
+
+  return <h1>Post Slug: {slug}</h1>
+}
+```
+
+## Server Rendering
+
+Next.js has built-in support for [Server-side Rendering](/docs/basic-features/pages#server-side-rendering.md). This means you can remove any instances of `StaticRouter` in your code.
+
+## Code Splitting
+
+Next.js has built-in support for [Code Splitting](https://reactrouter.com/web/guides/code-splitting). This means you can remove any instances of:
+
+- `@loadable/server`, `@loadable/babel-plugin`, and `@loadable/webpack-plugin`
+- Modifications to your `.babelrc` for `@loadable/babel-plugin`
+
+Each file inside your `pages/` directory will be code split into its own JavaScript bundle during the build process. Next.js [also supports](/docs/basic-features/supported-browsers-features.md#javascript-language-features) ES2020 dynamic `import()` for JavaScript. With it you can import JavaScript modules dynamically and work with them. They also work with SSR.
+
+For more information, read about [Dynamic Imports](https://nextjs.org/docs/advanced-features/dynamic-import).
+
+## Scroll Restoration
+
+Next.js has built-in support for [Scroll Restoration](https://reactrouter.com/web/guides/scroll-restoration). This means you can remove any custom `ScrollToTop` components you have defined.
+
+The default behavior of `next/link` and `next/router` is to scroll to the top of the page. You can also [disable this](https://nextjs.org/docs/api-reference/next/link#disable-scrolling-to-the-top-of-the-page) if you prefer.
+
+## Learn More
+
+For more information on what to do next, we recommend the following sections:
+
+<div class="card">
+  <a href="/docs/routing/introduction.md">
+    <b>Routing:</b>
+    <small>Learn more about routing in Next.js.</small>
+  </a>
+</div>
+
+<div class="card">
+  <a href="/docs/routing/dynamic-routes.md">
+    <b>Dynamic Routes:</b>
+    <small>Learn more about the built-in dynamic routes.</small>
+  </a>
+</div>
+
+<div class="card">
+  <a href="/docs/api-reference/next/link.md">
+    <b>Pages:</b>
+    <small>Enable client-side transitions with next/link.</small>
+  </a>
+</div>