docs(api): document GraphIngestor and ServiceIngestor SDK contract (NVBug 6628849)

[kheiss-uwzoo]

Summary

• Document create_ingestor() return types by run mode: GraphIngestor for inprocess and batch, ServiceIngestor for service.
• Generate API entries for the concrete classes and 26.08 generation and answer types instead of the stub Ingestor interface.
• Align the document-ingestion workflow page with the same factory contract.

Fixes NVBug 6628849.

Test plan

• mkdocs build --strict on this diff (local leftover untracked pages are not in this change)
• Confirm published API Reference headings for GraphIngestor, ServiceIngestor, Retriever.answer() types, and generation operators
• Confirm create_ingestor() is no longer described as graph-only on the published page

pre-draft: leakage, mkdocs --strict, ::a, ::p, ::r on the diff vs main

Base: upstream/main
Files: docs/docs/extraction/nemo-retriever-api-reference.md, docs/docs/extraction/workflow-document-ingestion.md

Check	Result
Leakage (page roles + see [ CTAs)	PASS — no see [ CTAs; faq/overview/multimodal-extraction nimOperator/nvcr.io/nim/installFfmpeg unchanged
Allowed paths	PASS — 2 documentation files
mkdocs --strict	PASS — exit 0 for this diff; local leftover untracked pages fail strict and are not in this change
::a audit	PASS — factory return types and listed methods match RC6 source (ingestor/core.py, graph_ingestor.py, service_ingestor.py, models/llm, operators/generation); generated create_ingestor summary hidden because source docstring is still graph-only
::p polish	Applied — factory table, import paths, and generated class sections; backticked GraphIngestor in method list
::r style	95% — no blocking issues; remaining source docstring is code drift, not this docs PR

Code drift (not in this docs PR): nemo_retriever/src/nemo_retriever/ingestor/core.py create_ingestor() docstring still says Graph-only ingestion factory, and the annotation still returns Ingestor.

PR: #2577