Migrate PDF Processing Core from Marker to Docling

Title: Migrate PDF Processing Core from Marker to Docling

Description:

We are currently using Marker as the core PDF parsing module in our system. This issue aims to **replace Marker with Docling as the main PDF parsing engine.

The updated implementation must retain the same functionality and output structure as the current Marker-based version to ensure backward compatibility with downstream processing components in Omniparse.

Requirements

• Replace Marker with Docling in the PDF parsing core.
• Ensure the output format is identical to what Marker currently produces (or provide a compatibility adapter).
• All existing test cases for Marker must pass with Docling.
• Provide a Google Colab notebook demonstrating the updated implementation and validating its output with test PDFs.
• Ensure performance is comparable or better than Marker in terms of speed and memory usage.

️ Tips

• Check out Docling's segment and node extraction tools—they should map closely to Marker’s annotation and token-level representations.
• You may need to write a thin compatibility layer to normalize Docling outputs to Marker-style structures.

✅ Acceptance Criteria

• Functionality parity with Marker: same sections, headers, paragraphs, tokens.
• Tests green ✅ in CI.
• Colab notebook demo included and reproducible.