docs: add guide for Next.js

[SuYaSh-PaThAk04]

Fixes #1334

Description

This PR adds a complete integration guide showing how to set up Lingo.dev CLI with Next.js (App Router) using react-intl. The guide walks through a clean setup of i18n support, configuring Lingo.dev synchronization, and rendering localized strings inside a Next.js app.

Files Added

docs/nextjs-app-routes.md — New integration tutorial for Next.js App Router

Testing Notes

Followed all steps on a fresh Next.js 15 project using create-next-app@latest

Verified language switching and translations with react-intl

Tested Lingo.dev CLI commands: lingo.dev pull, lingo.dev push, and lingo.dev fix

Some Screenshots of the guide

[davidturnbull]

Hi 👋

Thanks for the submission. I've left a few comments and assigned myself to review the PR in more depth and will get to it shortly.

Thanks. 😄

[SuYaSh-PaThAk04]

Hi 👋

Thanks for the submission. I've left a few comments and assigned myself to review the PR in more depth and will get to it shortly.

Thanks. 😄

Hi 👋 All the suggested fixes have been implemented as well.

Thanks for reviewing the PR

[SuYaSh-PaThAk04]

@davidturnbull Can you please re-review this PR and guide me if there are any other changes that should be made