Add comprehensive P9ML cognitive system documentation

[Copilot]

This PR adds the missing P9ML-README.md documentation that was referenced in issue #2 but not present in the repository. The P9ML (Plan 9 Machine Learning) system is already implemented throughout the codebase with extensive OpenCog cognitive architecture integration, but lacked comprehensive user documentation.

What's Added

P9ML-README.md (13KB, 506 lines)

A comprehensive guide covering the P9ML cognitive reasoning system with:

• Architecture Overview: Complete explanation of neural-symbolic AI integration with 3 mermaid diagrams
• Quick Start Guide: Installation and basic usage examples
• API Documentation: Full REST endpoint reference with code examples
• Development Guide: Custom operations, GGML kernel development, and extensions
• Advanced Features: Distributed reasoning, learning algorithms, and neural-symbolic fusion
• Integration Examples: Chat enhancement and knowledge base integration

Documentation Integration

• Updated DOCUMENTATION_INDEX.md to include P9ML documentation
• Updated main README.md to reference P9ML cognitive system features
• Ensured proper linking with existing OpenCog integration documentation

Technical Details

The P9ML system provides:

• AtomSpace Memory: Tensor-backed knowledge representation
• PLN Reasoning: Probabilistic Logic Networks with GGML acceleration
• ECAN Attention: Economic attention allocation for cognitive resources
• API Endpoints: RESTful interface for cognitive operations (/cognitive/*)
• GPU Acceleration: CUDA/Vulkan support for cognitive computations

Example usage:

# Enable P9ML cognitive system
koboldcpp --model model.gguf --p9ml --cognitive-memory 1024

# Use cognitive reasoning API
response = requests.post("http://localhost:5001/cognitive/reasoning", 
                        json={"premises": ["All cats are mammals"], 
                              "method": "deductive"})

The documentation complements the existing implementation found in expose.cpp, koboldcpp.py, and the opencog/ directory, providing users with the guidance needed to utilize the advanced cognitive capabilities already built into the system.

Fixes #2.

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