feat(config): add global document_timeout setting

[archasek]

1. Add support for document_timeout docling setting. Can be set globally (env var) or in the request payload.
2. Move .env from docling_serve folder to root.
3. Fix error handling, as there was serialization problem with the old code.
   API - When there's timeout, this kind of response (500) is returned:

{
    "detail": {
        "errors": [
            {
                "component_type": "document_backend",
                "module_name": "NoneType",
                "error_message": "Page 4 failed to parse."
            },
            {
                "component_type": "document_backend",
                "module_name": "NoneType",
                "error_message": "Page 5 failed to parse."
            },
            ...

Gradio:

Logs:

WARNING:        2025-02-19 01:23:06,856 - docling.pipeline.base_pipeline - Document processing time (8.141 seconds) exceeded the specified timeout of 3.000 seconds
INFO:     127.0.0.1:53490 - "POST /v1alpha/convert/file HTTP/1.1" 500 Internal Server Error
ERROR:  2025-02-19 01:23:07,164 - docling_serve.gradio_ui - Error processing file: No documents were generated by Docling.

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[dolfim-ibm]

shouldn't this be simple .env? otherwise we force the user on the given location.

[archasek]

Yeah, I also think there should be a single .env file in the root folder for the entire project. However, I noticed that there is already a .env.example file in the docling_serve folder. So, I decided not to change the current design at this point. What do you think?

[dolfim-ibm]

I think it is better with a single .env. Let's align all options, and in case use prefixes.

[archasek]

Done, please have a look dolfim. I also noticed an issue with error handling and fixed that (PR description updated). LMK if it's acceptable.

[dolfim-ibm]

On the single document, this makes perfect sense.
But this might not be the expected behavior when the user provides multiple document.

In this proposal, if 10 documents are provided and 1 fails, it will raise the error and produce no results.

In the latter, it would be better to have some manifest response file in the zip, where the system lists the result for each file.