Transfer docling integration

[julian-risch]

Summary and motivation

We transfer the existing integration from https://github.com/docling-project/docling-haystack

Checklist

If the request is accepted, ensure the following checklist is complete before closing this issue.

Tasks for new integration

• The code is documented with docstrings and was merged in the main branch
• Docs are published at https://docs.haystack.deepset.ai/
• There is a Github workflow running the tests for the integration nightly and at every PR
• A new label named like integration:<your integration name> has been added to the list of labels for this repository
• The labeler.yml file has been updated
• The package has been released on PyPI. Should be a new major release, although backward compatibility is ensured.
• An updated integration tile with a usage example has been added to https://github.com/deepset-ai/haystack-integrations
• The integration has been listed in the Inventory section of this repo README
• The feature was announced through social media
• Code coverage badges are shown in the README inventory : https://github.com/deepset-ai/haystack-core-integrations/pull/new/docling-coverage