docs: document DOCLING_NUM_THREADS environment variable

[Elettrotecnica]

Myself and some other user were wondering how one can increase the number of threads when processing a document. It turns out this is already possible, just undocumented.

All the best

Issue resolved by this Pull Request:
Resolves #257

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[dolfim-ibm]

FYI the DOCLING_NUM_THREADS option will be used only on cpu-mode, and it propagates to the number threads used when running the torch models.

[dolfim-ibm]

lgtm

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Documentation updates

Checked 2 published document(s). No updates required.

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