Add comprehensive documentation and code comments

This commit adds extensive documentation to the Mistral OCR CLI project:

- Add API.md with detailed API response format documentation
- Add CHANGELOG.md to track version changes
- Add CONTRIBUTING.md with guidelines for contributors
- Enhance README.md with more detailed usage examples and troubleshooting
- Add proper docstrings to all Python modules and functions
- Update requirements.txt with development dependencies
- Improve setup.py with better metadata

These changes make the project more accessible to users and contributors.

mistral_ocr/__main__.py

@@ -4,6 +4,14 @@ import os
 from mistral_ocr.commands import process, convert, markdown, version
 
 def main():
+    """
+    Main entry point for the Mistral OCR CLI.
+    
+    Parses command line arguments and dispatches to the appropriate command handler.
+    
+    Returns:
+        int: Exit code (0 for success, 1 for error)
+    """
     parser = argparse.ArgumentParser(
         description="A CLI tool for performing OCR on documents using Mistral AI.",
         prog="mistral-ocr"