feat: Add comprehensive XML support with structured Markdown conversion

[ramomen]

XML Support Enhancement

This PR adds comprehensive XML support to MarkItDown.

Changes

• Added general XML file support
• Enhanced RSS/Atom feed handling
• Improved error messages
• Updated documentation

Documentation

• Updated README.md with XML support details
• Added docstring documentation
• Included usage examples

Fixes #288

[ramomen]

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[afourney]

Looks awesome. Can you add some unit tests, and fix the formatting per the pre-commit hooks?