chore(examples): updated the example of with-react-jss to utlize the App router

[Manoj-M-S]

This PR updates the with-react-jss example to use the App Router. Here are the changes that have been made:

• Renamed the pages folder to the app folder.
• Converted the index.tsx to page.tsx file as part of the App Router.
• Created layout.tsx in app directory, merging functionality from both _app.tsx and _document.tsx:
  • Wraps content with StyledComponentsRegistry
• Created app/registry.tsx for JSS setup:
  • Moved JSS Provider and SheetsRegistry from _document.tsx
  • Moved style cleanup logic from _app.tsx
  • Handles server-side style generation
• Updated the package.json file.

The following actions were performed as part of this PR:

• Ran pnpm prettier-check with no issues found.
• Executed the pnpm check-examples script.

CC: @samcx

[ijjk]

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