feat: add Docling document converter

[vagenas]

Part of #3097

Proposed Changes:

Adding Docling document converter.

How did you test it?

(cd integrations/docling && hatch run test:all)

Notes for the reviewer

• Current import path of distribution package is from docling_haystack.converter import DoclingConverter.
• Import path introduced in this PR is from haystack_integrations.components.converters.docling import DoclingConverter

=> Presumably the PyPI package will become a shim to accordingly redirect the exposed packages for backwards compatibility?

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test:.

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[julian-risch]

Thank you so much @vagenas for the great and super quick collaboration!
I added backward compatibility for the old import paths and a test for that. In addition, I slightly changed how serialization works. There was a small issue. I ensure backward compatibility for deserialization too.
I will transfer the open GitHub issues from the docling repository and will comment on the open PRs there too. Feel free to archive the docling repo and point to haystack-core-integrations.