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/commands/markdown.py

@@ -5,6 +5,19 @@ from pathlib import Path
 from mistral_ocr.commands import process, convert
 
 def run(args):
+    """
+    Main entry point for the markdown command.
+    
+    Processes a document with OCR and converts the results to Markdown in one step.
+    This is a convenience command that combines the functionality of the 'process'
+    and 'convert' commands.
+    
+    Args:
+        args: Command line arguments parsed by argparse
+        
+    Raises:
+        SystemExit: If an error occurs during processing or conversion
+    """
     # Ensure that if --images is true, include_image_base64 is also true
     include_image_base64 = args.images