docs: add Docling loader docs (#29104)
### Description
This adds the docs for the Docling document loader.
[Docling](https://github.com/DS4SD/docling) parses PDF, DOCX, PPTX,
HTML, and other formats into a rich unified representation including
document layout, tables etc., making them ready for generative AI
workflows like RAG.
Some references:
- https://research.ibm.com/blog/docling-generative-AI
-
https://www.redhat.com/en/blog/docling-missing-document-processing-companion-generative-ai
- [Docling Technical Report](https://arxiv.org/abs/2408.09869)
The introduced `DoclingLoader` enables users to:
- use various document types in their LLM applications with ease and
speed, and
- leverage Docling's rich representation for advanced, document-native
grounding.
### Issue
Replacing PR #27987 as discussed with @efriis
[here](https://github.com/langchain-ai/langchain/pull/27987#issuecomment-2489354930).
### Dependencies
None
---------
Signed-off-by: Panos Vagenas <35837085+vagenas@users.noreply.github.com>
