chore(llm): add a basic LLM/AI coding instruction file (apache#290)

PR apache#293 follow it to create first step

Author: imbajin

hugegraph-llm/basic-introduction.md

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+# Basic Introduction
+
+This file provides guidance to AI coding tools and developers when working with code in this repository.
+
+## Project Overview
+
+HugeGraph-LLM is a comprehensive toolkit that bridges graph databases and large language models, 
+part of the Apache HugeGraph AI ecosystem. It enables seamless integration between HugeGraph and LLMs for building 
+intelligent applications with three main capabilities: Knowledge Graph Construction, Graph-Enhanced RAG, 
+and Text2Gremlin query generation.
+
+## Tech Stack
+
+- **Language**: Python 3.10+ (uv package manager required)
+- **Framework**: FastAPI + Gradio for web interfaces
+- **Graph Database**: HugeGraph Server 1.5+
+- **LLM Integration**: LiteLLM (supports OpenAI, Ollama, Qianfan, etc.)
+- **Vector Operations**: FAISS, NumPy, and will support multiple Vector DB soon
+- **Code style**: ruff & mypy (on the way, soon)
+- **Key Dependencies**: hugegraph-python-client
+
+## Essential Commands
+
+### Running the Application
+```bash
+# Install dependencies and create virtual environment (uv already installed)
+uv sync
+# Activate virtual environment
+source .venv/bin/activate
+# Launch main RAG demo application
+python -m hugegraph_llm.demo.rag_demo.app
+# Custom host/port
+python -m hugegraph_llm.demo.rag_demo.app --host 127.0.0.1 --port 18001
+```
+
+### Testing
+```bash
+pytest src/tests/
+# Or using unittest
+python -m unittest discover src/tests/
+```
+PS: we skip Docker Deployment details here.
+
+## Architecture Overview
+
+### Core Directory Structure
+- `src/hugegraph_llm/api/` - FastAPI endpoints (rag_api.py, admin_api.py)
+- `src/hugegraph_llm/demo/rag_demo/` - Main Gradio UI application
+- `src/hugegraph_llm/operators/` - Core processing pipelines
+- `src/hugegraph_llm/models/` - LLM, embedding, reranker implementations
+- `src/hugegraph_llm/indices/` - Vector and graph indexing
+- `src/hugegraph_llm/config/` - Configuration management
+- `src/hugegraph_llm/utils/` - Utilities, logging, decorators
+
+### Key Processing Pipelines
+
+1. **KG Construction** (`operators/kg_construction_task.py`)
+   - Text chunking and vectorization pipeline
+   - Schema management and validation
+   - Information extraction using LLMs
+   - Graph data commitment to HugeGraph
+
+2. **Graph RAG** (`operators/graph_rag_task.py`)
+   - Multi-modal retrieval (vector, graph, hybrid)
+   - Keyword extraction and entity matching
+   - Graph traversal and Gremlin query generation
+   - Result merging and reranking
+
+3. **Text2Gremlin** (`operators/gremlin_generate_task.py`)
+   - Natural language to Gremlin query conversion
+   - Template-based and few-shot learning approaches
+
+### Configuration Management
+
+- Main config: `.env` file (generate with `config.generate` module)
+- Prompt config: `src/hugegraph_llm/resources/demo/config_prompt.yaml`
+- HugeGraph connection settings in environment variables
+- LLM provider configuration through `LiteLLM` & `openai/ollama` client
+
+## Development Workflow
+
+1. **Prerequisites**: Ensure HugeGraph Server is running and LLM provider is configured
+2. **Environment Setup**: Use UV for dependency management, activate virtual environment
+3. **Configuration**: Generate configs and set up .env file with proper credentials
+4. **Development**: Use Gradio demo for interactive testing, FastAPI for programmatic access
+5. **Testing**: Unit tests use standard unittest framework in src/tests/
+
+## Important Notes
+
+- Always use `uv` package manager instead of `pip` for dependency management
+- HugeGraph Server must be accessible while running the app
+- The system supports multiple LLM providers through `LiteLLM` abstraction
+- Each file should be better < 600 lines for maintainability