feat: make an experimental outline serializer

[PeterStaar-IBM]

No description provided.

[mergify]

Merge Protections

Your pull request matches the following merge protections and will not be merged until they are valid.

🔴 Require two reviewer for test updates

This rule is failing.

When test data is updated, we require two reviewers

• #approved-reviews-by >= 2

🟢 Enforce conventional commit

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Make sure that we follow https://www.conventionalcommits.org/en/v1.0.0/

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

[github-actions]

✅ DCO Check Passed

Thanks @PeterStaar-IBM, all your commits are properly signed off. 🎉

[codecov]

Codecov Report

❌ Patch coverage is 76.23762% with 24 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
docling_core/experimental/serializer/outline.py	76.23%	24 Missing ⚠️

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