docs: migrate from MkDocs to Fern

[yamini]

Summary

Migrates the NeMo Platform documentation from MkDocs (Material) to Fern MDX, the new required docs format. Converts all 188 published pages, scaffolds the Fern site under docs/fern/, wires the REST API reference natively from the OpenAPI spec, and preserves the current launch gating.

Draft for review — not for merge yet. Local preview + fern check + fern docs broken-links all pass. A few follow-ups are listed below (notably the Fern org theme access and docs CI migration).

What changed

• 188 pages converted from MkDocs Material/pymdown to Fern MDX: admonitions → <Note>/<Warning>/..., === "tab" → <Tabs>/<Tab>, --8<-- snippets inlined, {{ macro }} variables substituted, and MDX-unsafe braces/HTML comments escaped.
• Nested layout (the preferred NeMo pattern, per Automodel #2291): authored MDX lives at docs/<section>/<page>.mdx; Fern config/theme/components/nav live under docs/fern/.
• Native API reference: the OpenAPI spec is registered at docs/fern/apis/nemo-platform/ and rendered as an interactive, per-endpoint API reference (replaces the old <swagger-ui> embed, which doesn't work in Fern).
• Launch gating preserved: the sections the MkDocs hide_unready_docs hook hides (Platform, Fine-tune Models/Customizer, Synthesize Safe Data/Safe Synthesizer, Example Applications, plus gated paths like auth/**, evaluator/benchmarks/**) are marked hidden: true — out of the sidebar but still routable, so cross-links resolve and there are no 404s. Same visible surface as the current site.
• Stable URLs: explicit slug: on every nav entry pins clean slugs (the Fern CLI otherwise splits CamelCase/acronyms, e.g. NemoGuard NIMs → nemo-guard-ni-ms).
• Redirects carried over from the old MkDocs redirects plugin.
• llms.txt: served automatically by Fern (/llms.txt, /llms-full.txt) — no extra files needed.
• Removed the MkDocs apparatus: mkdocs.yml, docs/_hooks, docs/_overrides, theme assets, and the excluded template/, work/, notebooks/ trees.

Reviewing the diff

Commits are split so each page is reviewable:

1. rename — git mv docs/*.md → docs/*.mdx (no content change; pure renames).
2. convert — the actual MDX conversion + docs/fern/ scaffold.

So git show <convert> -- docs/<page>.mdx shows the conversion, and git log --follow docs/<page>.mdx reaches the original .md history.

Preview locally (hot-reload, like mkdocs serve)

npm install -g fern-api        # once
cd docs/fern
fern docs dev                  # http://localhost:3000, hot-reload

fern check and fern docs broken-links validate config and links (these are what CI should run).

Verification

• fern check → 0 errors
• fern docs broken-links → all checks passed (0 broken)
• Visual spot-checks: homepage, a Tabs page (manage-files — tab widget renders), an images page (multimodal-data — image renders), and the generated API reference (per-endpoint pages) all render.
• No unconverted MkDocs syntax remains (check_unconverted clean across docs/ — no leftover !!! admonitions, === "tab" blocks, ??? details, or MyST roles). The only 2 flags are literal {{ auth_secret }} Jinja examples inside inline code in cli/reference.mdx, which render correctly as intended.

Follow-ups before merge

• Fern org theme access — docs/fern/docs.yml currently uses a minimal NVIDIA-green theme so the site builds/previews for anyone. To get the full NVIDIA chrome, the publishing Fern account/org must be granted access to the NVIDIA/fern-components global theme (otherwise fern docs dev/generate returns HTTP 403). Then replace the colors block with global-theme: nvidia (documented inline).
• Docs CI migration — docs.yaml (and the docs job in ci.yaml) still build MkDocs and will fail. Replace with the Fern CI workflows (fern-docs-ci.yml etc.), which need a DOCS_FERN_TOKEN secret for preview/publish.
• Asset de-dup — original MkDocs image/asset dirs under docs/ remain alongside the docs/fern/ copies Fern serves. Safe to remove (all image refs are absolute → docs/fern/); left in place pending a visual confirmation pass.
• Notebooks — the .ipynb tutorials (Customizer, Example Applications) are all in currently-gated sections and were not converted; wire NotebookViewer when those sections un-gate.

🤖 Generated with Claude Code

[github-actions]

Suite	Lines Covered	Line Rate	Branch Rate
Unit Tests	N/A	N/A	N/A
Integration Tests	N/A	N/A	N/A