docs: Add Case Study 02 to Documentation and Update Parser Usage

[AdemBoukhris457]

Summary

This MR adds the case_study_02_scanned_document_restoration notebook to the documentation tables and updates the notebook to use the appropriate parser for already-restored PDFs.

Changes Made

Documentation Updates

• ✅ Added case_study_02_scanned_document_restoration to Interactive Notebooks table in README.md
• ✅ Added case_study_02_scanned_document_restoration to Interactive Notebooks table in docs/en/index.md
• ✅ Added case_study_02_scanned_document_restoration to Interactive Notebooks table in docs/zh/index.md

Notebook Improvements

• ✅ Updated PDF parsing section to use StructuredPDFParser for already-restored PDFs instead of EnhancedPDFParser
  • This avoids redundant restoration since the PDF is already processed by DocRes engine
  • Improves parsing efficiency and accuracy

Context

The Case Study 02 notebook demonstrates:

• DocRes image restoration capabilities across all 6 restoration tasks (appearance, dewarping, deshadowing, deblurring, binarization, end2end)
• Comparison of PDF parsing results before and after restoration
• Visual comparison of original vs restored document pages

Testing

• Verified notebook runs successfully
• Confirmed documentation links are correct
• Tested parser changes with restored PDFs

Related

• Part of ongoing documentation improvements
• Enhances discoverability of restoration capabilities