feat: Integrate docling-hierarchical-pdf back into docling

[krrome]

This is still a draft with limited functionality (and failing tests) to gauge whether my approach of the integration is in line with the docling team. I will keep extending the PR to full functionality, but I would like to receive feedback on the integration as early as possible.

Changes:

• The reading order model was extended to handle header hierarchies.
• docling/models/header_hierarchy was added as a home to header level inference

Issue resolved by this Pull Request:
Resolves #2591, #652, #287, #1023, #2121 and maybe more.

Checklist:

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

[github-actions]

❌ DCO Check Failed

Hi @krrome, your pull request has failed the Developer Certificate of Origin (DCO) check.

This repository supports remediation commits, so you can fix this without rewriting history — but you must follow the required message format.

---

🛠 Quick Fix: Add a remediation commit

Run this command:

git commit --allow-empty -s -m "DCO Remediation Commit for Roman Kayan BAZG <roman.kayan@bazg.admin.ch>

I, Roman Kayan BAZG <roman.kayan@bazg.admin.ch>, hereby add my Signed-off-by to this commit: 36c6432505080aaa3b972ea0400256755d01a74d
I, Roman Kayan BAZG <roman.kayan@bazg.admin.ch>, hereby add my Signed-off-by to this commit: f8bd273f216eb8e1c72d658c80f8363d9cfee456
I, Roman Kayan BAZG <roman.kayan@bazg.admin.ch>, hereby add my Signed-off-by to this commit: 471a0ce5cb547f81c0b087e21e2503cd1210362a
I, Roman Kayan BAZG <roman.kayan@bazg.admin.ch>, hereby add my Signed-off-by to this commit: c4073956643fd356fdf3cb95d8fbefb5468d482f"
git push

---

🔧 Advanced: Sign off each commit directly

For the latest commit:

git commit --amend --signoff
git push --force-with-lease

For multiple commits:

git rebase --signoff origin/main
git push --force-with-lease

More info: DCO check report

[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

Wonderful, this rule succeeded.

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)(?:\(.+\))?(!)?: