docs: add Nebius Serverless Endpoint deployment example

[SalikovAlex]

Add a Nebius Serverless Endpoint deployment guide, configuration, and asynchronous client for exporting Markdown and Docling JSON to private Object Storage. Link the example from the deployment index.

The client records a receipt before submission, never replays an ambiguous conversion POST, and supports resuming observation of an acknowledged task. Downloads are bounded and restricted to the expected storage host, bucket, and task prefix; a completion manifest is written only after both artifacts pass validation. The guide explains that exported objects survive independently while the local engine's task registry is lost on process replacement.

Validation:

• 40 client tests passed on Python 3.10 and 3.12; all six pre-commit hooks passed.
• 77 existing API/health/policy/form tests passed with session loop-scope overrides for the existing pytest-asyncio fixture mismatch.
• Real CPU app/OpenAPI and conversion tests passed with Moto S3 export, checksum readback, and injected export failure.
• Live validation on an L40S in eu-north1 covered CUDA startup, native ingress/SecretStash/S3, text/table and scanned PDFs, separate read-only artifact verification, authentication, input limits, export failure, disconnect/deadline resume, stop/start, and confirmed resource cleanup. An invalid image was rejected before provisioning.

The live run established the CLI's text receipt formats and identified ocr_preset=easyocr as the effective multipart option; both are reflected in the example. Production capacity/ingress limits and full one-hour application URL expiry remain unmeasured. This adds no parser/runtime changes or package dependencies.

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