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

[mergify]

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• title ~= ^(fix|feat|docs|style|refactor|perf|test|build|ci|chore|revert)(?:\(.+\))?(!)?:

[codecov]

Codecov Report

❌ Patch coverage is 93.97590% with 10 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
docling_core/types/doc/document.py	92.24%	9 Missing ⚠️
docling_core/transforms/serializer/markdown.py	96.29%	1 Missing ⚠️

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[github-actions]

✅ DCO Check Passed

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