Add doughnut http endpoint

[edknv]

Description

Part of https://github.com/NVIDIA/nv-ingest-private/issues/52 Also closes https://github.com/NVIDIA/nv-ingest-private/issues/49.

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.

[edknv]

321052f adds support for preserving the text bounding boxes in the metadata (in the hierarchy field) and addresses issue https://github.com/NVIDIA/nv-ingest-private/issues/49. It also changes the way text blocks are concatenated to concatenate text blocks with \n\n as requested by research team.

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