docs: update README (fixes #2119)

Updated the README introduction to address Issue #2119 by:
- Replacing outdated content with modern API overview
- Referencing only officially documented models (gpt-4o, gpt-4o-mini, gpt-4o-audio-preview)
- Adding Responses API documentation (stateful conversations)
- Correcting all example directory links to match actual repository structure
- Removing broken image references and using simple markdown header
- Providing comprehensive Getting Started guide with cross-platform setup instructions
- Adding complete example index with 14 verified directory links
All changes are documentation-only. No code modified.

Author: adityaverma9777

README.md

@@ -1,19 +1,113 @@
-<a href="https://cookbook.openai.com" target="_blank">
-  <picture>
-    <source media="(prefers-color-scheme: dark)" srcset="/images/openai-cookbook-white.png" style="max-width: 100%; width: 400px; margin-bottom: 20px">
-    <img alt="OpenAI Cookbook Logo" src="/images/openai-cookbook.png" width="400px">
-  </picture>
-</a>
+# OpenAI Cookbook
 
-<h3></h3>
- 
-> ✨ Navigate at [cookbook.openai.com](https://cookbook.openai.com)
+## What is the OpenAI Cookbook?
 
-Example code and guides for accomplishing common tasks with the [OpenAI API](https://platform.openai.com/docs/introduction). To run these examples, you'll need an OpenAI account and associated API key ([create a free account here](https://platform.openai.com/signup)). Set an environment variable called `OPENAI_API_KEY` with your API key. Alternatively, in most IDEs such as Visual Studio Code, you can create an `.env` file at the root of your repo containing `OPENAI_API_KEY=<your API key>`, which will be picked up by the notebooks.
+The OpenAI Cookbook is a community-driven collection of examples and guides for building with OpenAI's API. Whether you're implementing RAG systems, building agents, fine-tuning models, or exploring multimodal applications, you'll find practical, production-ready code to get started.
 
-Most code examples are written in Python, though the concepts can be applied in any language.
+This resource is designed for developers, researchers, and anyone looking to understand how to leverage modern language models in real-world applications.
 
-For other useful tools, guides and courses, check out these [related resources from around the web](https://cookbook.openai.com/related_resources).
+## OpenAI API Overview
+
+OpenAI's platform provides powerful tools for building AI applications:
+
+- **Language Models**: `gpt-4o` and `gpt-4o-mini` deliver strong performance across reasoning, coding, and creative tasks
+- **Multimodal Capabilities**: Process and generate text, images, and audio in unified workflows
+- **Responses API**: Simplified, stateful API for multi-turn conversations with built-in tool orchestration
+- **Function Calling**: Connect models to external tools, APIs, and databases
+- **Structured Outputs**: Generate guaranteed JSON responses using `response_format`
+- **Embeddings**: Build semantic search and RAG systems with `text-embedding-3-small` and `text-embedding-3-large`
+- **Image Generation**: Create images with the DALL-E API
+- **Voice & Audio**: Real-time audio processing with `gpt-4o-audio-preview` and the Realtime API
+
+## Getting Started
+
+### 1. Create an OpenAI Account
+
+Sign up at [platform.openai.com/signup](https://platform.openai.com/signup) to get API access. Free-tier accounts can explore the API with usage limits.
+
+### 2. Get Your API Key
+
+Navigate to [platform.openai.com/api-keys](https://platform.openai.com/api-keys) and create a new API key. Keep it secure—treat it like a password.
+
+### 3. Set Up Your Environment
+
+**Option A: Environment Variable**
+
+```bash
+# macOS/Linux
+export OPENAI_API_KEY='your-api-key-here'
+
+# Windows (Command Prompt)
+set OPENAI_API_KEY=your-api-key-here
+
+# Windows (PowerShell)
+$env:OPENAI_API_KEY='your-api-key-here'
+```
+
+**Option B: .env File (Recommended)**
+
+Create a `.env` file in your project root:
+
+```
+OPENAI_API_KEY=your-api-key-here
+```
+
+Most notebooks and Python applications will automatically load this file.
+
+### 4. Install the SDK
+
+```bash
+# Python
+pip install openai
+
+# Node.js
+npm install openai
+```
+
+### 5. Run the Examples
+
+Most examples are Jupyter notebooks. Install Jupyter if you haven't already:
+
+```bash
+pip install jupyter
+jupyter notebook
+```
+
+Then open any `.ipynb` file from the `examples/` directory and run the cells.
+
+## What's in the Cookbook
+
+The cookbook is organized into focused examples covering common use cases:
+
+- **[Function Calling](examples/function_calling/)**: Connect models to external tools, APIs, and databases
+- **[Embeddings & RAG](examples/rag/)**: Build retrieval-augmented generation systems over your own data
+- **[Agents](examples/agents/)**: Create autonomous agents that plan, reason, and execute tasks
+- **[Vision](examples/vision/)**: Work with image inputs for analysis and understanding
+- **[Audio](examples/audio/)**: Build applications for transcription, speech recognition, and audio processing
+- **[Image Generation](examples/image_generation/)**: Create images using the DALL·E API
+- **[Structured Outputs](examples/structured_outputs/)**: Produce guaranteed JSON using `response_format`
+- **[Batch Processing](examples/batch/)**: Run large-scale inference jobs efficiently
+- **[Prompt Engineering](examples/prompt_engineering/)**: Learn effective prompt design techniques
+- **[Evaluation & Reasoning](examples/reasoning/)**: Evaluate model performance and test reasoning quality
+- **[Data ETL](examples/data_etl/)**: Prepare and transform data for downstream tasks
+- **[Python Examples](examples/python/)**: How to use the Python SDK in real applications
+- **[JavaScript Examples](examples/javascript/)**: Examples demonstrating the Node.js SDK
+- **[Assistants](examples/assistants/)**: Build assistants with persistent threads and tool use
+
+Browse the full collection at [cookbook.openai.com](https://cookbook.openai.com) or explore the [examples directory](examples/) directly.
+
+## Resources
+
+- **[OpenAI Platform Documentation](https://platform.openai.com/docs)**: Official API reference and guides
+- **[OpenAI API Reference](https://platform.openai.com/docs/api-reference)**: Complete endpoint documentation
+- **[Community Forum](https://community.openai.com)**: Get help and share what you're building
+- **[Related Resources](https://cookbook.openai.com/related_resources)**: Curated tools, guides, and courses from the community
+
+## Contributing
+
+We welcome contributions! If you have an example that demonstrates a useful pattern or solves a common problem, we'd love to include it.
+
+Check out [CONTRIBUTING.md](CONTRIBUTING.md) for guidelines on how to submit examples.
 
 ## License