Examples: Updated the example of catch all routes to utilize the App Router

[niketchandivade]

This PR updates the catch-all-routes example to use the App Router. Here are the changes that have been made:

• Renamed the "pages" folder to the "app" folder.
• Updated the routing for /, /about, and /post/[...slug] files to align with the App Router.
• Added the layout.tsx file as part of the App Router.
• Renamed used of next/router instead of used params
• Updated the package.json file.
• Updated the Readme file.

CC @samcx

[ijjk]

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[samcx]

Thank you for submitting a PR!