feat: implement serialization methods

[OGuggenbuehl]

This pull request enhances the DoclingConverter class by adding introducing serialization & deserialization methods. It also updates the corresponding tests to ensure the new functionality is well-covered.

Changes to DoclingConverter:

• Serialization and deserialization:
  • Introduced to_dict and from_dict methods to allow the DoclingConverter to be serialized and deserialized for pipeline persistence.

[mergify]

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