feat: rename distribution pkg & import pkg

[vagenas]

Rationale: aligning with the convention used in the langchain world, where pkgs are called langchain-X
See https://github.com/langchain-ai/langchain/blob/b1dafaef9b2c32151acb42367012e0a7d35067e3/libs/packages.yml

The repo itself though can remain as docling-langchain I think, as users do not really need to care about that name.

[mergify]

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