Add DocConverter for legacy .doc file support (fixes #23)

[sahaj24]

Summary

This PR adds support for legacy .doc (Microsoft Word 97-2003 binary format) files.

Fixes #23

Implementation

Added a DocConverter that uses a system tool to extract content from .doc files:

• macOS: textutil (built-in, no dependencies needed)
• Linux: libreoffice --headless --convert-to html or antiword

The converter produces HTML from the .doc file via the system tool, then passes it through the existing HtmlConverter to produce Markdown.

Changes

• New file: converters/_doc_converter.py - DocConverter implementation
• Modified: converters/__init__.py - Export DocConverter
• Modified: _markitdown.py - Register DocConverter in builtins
• Modified: pyproject.toml - Add optional [doc] dependency group
• New file: tests/test_files/test.doc - Test fixture
• Modified: tests/_test_vectors.py - Add test vector for .doc

Notes

The [doc] optional dependency is empty since the converter relies on system-installed tools rather than Python packages.

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