feat: Add PPTX notes slides

[maciejwie]

Presenter notes are a valuable part of a Powerpoint presentation and are worth extracting. Docling uses uses the python-pptx library for parsing Powerpoint pptx files, which supports reading from the presenter notes, and which are stored as notes slides.

Issue resolved by this Pull Request: Resolves #473

Checklist:

• [ ] Documentation has been updated, if necessary.
• [ ] Examples have been added, if necessary.
• [X] Tests have been added, if necessary.

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[PeterStaar-IBM]

@maciejwie I like your PR a lot, I just think we need an ability to distinguish between the regular text and the (invisible) text/notes.

My proposal is to first merge this (https://github.com/DS4SD/docling-core/pull/80) and then update this PR to explicitly tag it as InvisibleTextItem with the correct label.

[maciejwie]

Hi @PeterStaar-IBM, sounds good. I'm subscribed to that PR and see there's still some discussion about it, and when it gets merged I will update this one.

[PeterStaar-IBM]

@maciejwie I want to go fast on this. We are just merging in now a few big performance improvements (10x faster pdf-parsing, improved layout postprocessing and GPU acceleration). Once done, we will update this one together with another PR: basically, we would like to add the author notes as part of the furniture (yes, this is the correct term: https://en.wikipedia.org/wiki/Page_layout, I was also surprised).

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