adds flake file

[EstebanMarin]

Add Nix Flake Support

This PR adds Nix flake support to ensure consistent environment setup with the exact versions specified in the book.
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Changes

• Added flake.nix for reproducible development environment
• Automatically sets up Python 3.10, micromamba, and all dependencies from environment.yml
• Installs IPython kernel automatically on first run
• Added macOS Apple Silicon support via micromamba
• Updated README with Nix instructions
• Added INSTRUCTIONS.org with setup guide

Usage

nix develop
jupyter notebook

The flake handles environment creation, activation, and kernel installation automatically.