docs: Clarify pymupdf.layout import order for OCR support

[Deepaksaini00]

Summary

This PR improves the PyMuPDF4LLM documentation around OCR support. It clarifies how OCR actually works, what dependencies are required, and highlights an important import-order requirement that isn’t currently obvious from the docs.

Key points

• Documented that pymupdf.layout must be imported before pymupdf4llm for OCR heuristics to run
• Added an explanation of when and how OCR is triggered (image-only pages, unreadable text, partial OCR)
• Listed the full set of requirements (layout import, use_ocr, Tesseract, OpenCV)
• Added a small decision-flow diagram to make the behavior easier to understand

This is a documentation-only update — no code or runtime behavior is changed.

Fixes #4833

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