docs(fern): scaffold NeMo Retriever Fern docs from MkDocs

[kheiss-uwzoo]

Summary

• Scaffold Fern docs under docs/fern/ (NVIDIA global theme, nightly nav from MkDocs, redirects, MDX components).
• Convert published MkDocs pages to Fern MDX under docs/docs/**/*.mdx (MkDocs .md sources remain for dual-publish).
• Add Fern check / preview / publish GitHub Actions wired to docs/fern and docs/docs.

Notes

• fern check passes locally (cd docs/fern && fern check).
• External MkDocs nav links (GitHub quickstart / Helm / CLI / starter kits) are Fern link: entries.
• Follow-up: canonicalize internal link slugs; add latest.yml when freezing a GA train; confirm DOCS_FERN_TOKEN org secret before publish.

Test plan

• cd docs/fern && fern check
• Spot-check sidebar order against docs/mkdocs.yml nav:
• Preview a few pages (overview, deployment-options, multimodal-extraction) via Fern preview workflow or fern docs dev
• Confirm Fern CI path filters fire on docs/fern/** and docs/docs/**