docs: update README to reflect repository deprecation and migration details (#21)

Author: a-klos

README.md

@@ -1,4 +1,20 @@
-# RAG Core library
+# 🚨 REPOSITORY DEPRECATED 🚨
+
+## ⚠️ This repository is no longer maintained ⚠️
+
+### 📢 Important Notice
+
+**As of July 15, 2025, the contents of this library have been migrated to a new monorepo structure to simplify development, improve code reuse, and streamline CI/CD workflows.**
+
+### 🔄 Migration Information
+
+- **New Repository**: Development continues in the [`stackitcloud/rag-template`](https://github.com/stackitcloud/rag-template) repository
+- **Action Required**: All future updates, issues, and pull requests should be directed there
+- **Status**: This repo is preserved for historical reference only
+
+---
+
+# RAG Core Library
 
 This repository contains the core of the STACKIT RAG template.
 It provides comprehensive document extraction capabilities including support for files (PDF, DOCX, XML), web sources via sitemaps, and Confluence pages.