Add Docling dataset conversion timings

[mapo80]

Summary

• document conversion performance for Docling PDF, TIFF and PNG samples
• compare MarkItDownNet timings against markitdown benchmarks

Testing

• ~/.dotnet/dotnet build
• ~/.dotnet/dotnet test

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https://chatgpt.com/codex/tasks/task_e_689c22f8b69c83259ee3140bbd1c08f8

[Copilot]

Pull Request Overview

This PR updates the Docling dataset conversion timing benchmarks to include TIFF and PNG image samples, providing a more comprehensive performance comparison with markitdown. The updates reflect improved performance results and expand the test coverage beyond PDF files.

Key Changes:

• Added timing data for TIFF and PNG image file processing
• Updated PDF timing results with improved performance metrics
• Enhanced the comparison section to include image format timings against markitdown benchmarks

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[Copilot]

[nitpick] Missing Oxford comma before 'and PNG'. Should be 'PDF, TIFF, and PNG samples'.

Suggested change

	The following timings were captured while converting the PDF, TIFF and PNG samples from Docling's `tests/data` directory.
	The following timings were captured while converting the PDF, TIFF, and PNG samples from Docling's `tests/data` directory.