docs(extraction): add 26.08 release notes for post-26.05 changes

[kheiss-uwzoo]

Summary

• Replace the 26.05 release-notes body with customer-facing 26.08 / 26.8.0 highlights for changes that landed on main after the 26.05 scope.
• Cover OCR/object-detection stack defaults (OCR v2 everywhere, nemotron-object-detection:2.0.0, VL embed/rerank 2.3.0), Omni caption default, agentic retrieval (CLI/SDK/HTTP/MCP), service query enhancements, broader text ingest, LanceDB hybrid/autodetect, and related service/CLI packaging updates.
• Keep prior trains as previous-version links; retain the GitHub Pages RC readiness callout that docs.nvidia.com stays on the published 26.05 snapshot until a separate publish run.

Out of scope

• Broader 26.08 version-string cutover across README/Helm (see draft docs: rebrand main for 26.08 RC GitHub Pages QA review #2430)
• Runtime/Helm chart defaults (already on main; this PR documents them only)

Test plan

• Review release notes against recent main merges for accuracy (OCR/OD, agentic, service query, text ingest)
• Confirm no leftover 26.05-only claims under the 26.08 heading (e.g. Helm OCR v1 default)
• Spot-check previous-version links still resolve
• Coordinate with docs: rebrand main for 26.08 RC GitHub Pages QA review #2430 so GH Pages QA sees consistent 26.08 labeling after both merge