Enhance README and improve error handling for Document Intelligence

[dhruvgupta2109]

This pull request improves the usability and documentation of the MarkItDown project, focusing on enhanced documentation for its multi-package ecosystem, clearer usage instructions, and improved CLI behavior and testing for stdin and Document Intelligence scenarios.

Documentation improvements:

• Expanded the README.md to document all packages in the repository, their compatibility, and Python version support. Added a table summarizing each package and its requirements. [1] [2]
• Added advanced CLI usage examples and clarified how to use stdin, URLs, and data URIs, as well as how to convert from URI sources directly in Python. [1] [2]
• Added a troubleshooting section to the README.md to help users resolve common issues with plugins, optional dependencies, OCR, and MCP server deployment.

CLI and core functionality:

• Updated __main__.py to reject stdin (including - as filename) for Document Intelligence conversions, with a clear error message, while allowing stdin for standard conversions.

Testing enhancements:

• Added CLI tests to ensure that stdin via - works as expected and that Document Intelligence mode properly rejects stdin, improving reliability and preventing regressions.

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