Re-add _document to with-styled-components example (vercel#35163)

`_document.js` was removed from the styled components swc example
I have tested with and without the custom `_document.js`
Unfortunately without `_document.js` there is FOUC (Flash of unstyled content) on initial request.



## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `yarn lint`


Co-authored-by: JJ Kasper <[email protected]>

examples/with-styled-components/pages/_document.js

@@ -0,0 +1,30 @@
+import Document from 'next/document'
+import { ServerStyleSheet } from 'styled-components'
+
+export default class MyDocument extends Document {
+  static async getInitialProps(ctx) {
+    const sheet = new ServerStyleSheet()
+    const originalRenderPage = ctx.renderPage
+
+    try {
+      ctx.renderPage = () =>
+        originalRenderPage({
+          enhanceApp: (App) => (props) =>
+            sheet.collectStyles(<App {...props} />),
+        })
+
+      const initialProps = await Document.getInitialProps(ctx)
+      return {
+        ...initialProps,
+        styles: (
+          <>
+            {initialProps.styles}
+            {sheet.getStyleElement()}
+          </>
+        ),
+      }
+    } finally {
+      sheet.seal()
+    }
+  }
+}