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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[Deepaksaini00]

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[Deepaksaini00]

recheck

[Deepaksaini00]

@JorjMcKie Could you please review this PR?

[JorjMcKie]

Some general comments:

1. The OpenCV dependency has been removed. The corresponding checks are being done using the batteries already included.
2. We are in the process of re-configuring the components across our products pymupdf4llm, pymupdf and the layout plugin.

These activities will of course entail major documentation adjustments. Therefore, I'm afraid that your PR will become obsolete within a few weeks.