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.