[Docs] Next.js Application localization path corrected

[Axshatt]

🧩 Issue

The documentation link for the Next.js framework page was broken.
The old URL:

https://lingo.dev/en/compiler/frameworks/nextjs

was returning a 404 error.

🔍 Root Cause

The documentation structure was updated, and the correct path now includes the -app-router suffix:

https://lingo.dev/en/compiler/frameworks/nextjs-app-router

✅ Fix

Updated all references and links pointing to the old Next.js page to use the correct route.

🧠 Impact

• Fixed 404 redirection issue
• Ensures users can access the correct Next.js compiler documentation page
• Improves documentation accuracy and user experience