fix(markitdown-ocr): make PyMuPDF an optional dependency to fix AGPL licensing concern

[octo-patch]

Fixes #1675

Problem

markitdown-ocr declared PyMuPDF>=1.24.0 as a required dependency, but PyMuPDF is licensed under AGPL-3.0. Since the plugin itself is MIT-licensed, this mismatch was not disclosed anywhere. Any user who installed markitdown-ocr silently acquired an AGPL transitive dependency, which can affect the licensing requirements of applications that distribute the software.

PyMuPDF is only used in one place: a fallback path in _pdf_converter_with_ocr.py that handles malformed PDFs that pdfplumber cannot open (e.g. truncated EOF). This is an edge case, not the primary conversion path.

Solution

• Move PyMuPDF from dependencies to an optional [pymupdf] extra in pyproject.toml
• Add an [all] convenience extra that bundles both [llm] and [pymupdf]
• Add a clear license notice in README.md explaining the AGPL implications and showing how to install with or without PyMuPDF

The existing import fitz call is already inside a try/except, so the fallback path degrades gracefully when PyMuPDF is not installed — no code changes needed.

Testing

• pip install markitdown-ocr installs without PyMuPDF; standard PDF/DOCX/PPTX/XLSX conversion works normally
• pip install 'markitdown-ocr[pymupdf]' enables the malformed-PDF fallback
• pip install 'markitdown-ocr[all]' pulls in both openai and PyMuPDF extras