feat: add DocTags serializers

[vagenas]

• Introduced inline DocTag
• Using new serialization framework code seems to automatically fix some bugs around list handling

Still todo:

• Handle page breaks
• fix nested list items
• centralize token labels
• Fix page headers & footnotes not showing up in new export
• reconcile internally blocked item types (e.g. caption) with label parameter
• fix OTSL location tokens
• Propagate & use all parameters
• Move all relevant code out of DoclingDocument
• Adapt any moved code to framework & its conventions
• Iron out any misalignment with previous exporters
• cache excluded refs
• clarify escaping e.g. HTML
• add tests covering labels / layers params

[mergify]

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

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