feat: add metadata model hierarchy

[vagenas]

Main goal has been to establish metadata model that:

• allows both prescribed and custom fields, with a way to distinguish the two
• allows both common and type-specific fields, so that we can further specify the metadata reqs e.g. for PictureItem
• accounts for serialization and deserialization, e.g. ideally avoiding serializing additional data if no meta info available

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[codecov]

Codecov Report

❌ Patch coverage is 94.32624% with 16 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
docling_core/types/doc/document.py	93.07%	9 Missing ⚠️
docling_core/transforms/serializer/doctags.py	80.00%	4 Missing ⚠️
docling_core/transforms/serializer/html.py	92.30%	2 Missing ⚠️
docling_core/transforms/serializer/markdown.py	96.96%	1 Missing ⚠️

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[dosubot]

Documentation Updates

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^{How did I do? Any feedback?}  

[PeterStaar-IBM]

lgtm!

However, it would be good to update the init.py with the newly introduced classes.

[PeterStaar-IBM]

lgtm!