Migrate NeMo Run docs to Fern MDX

[lbliii]

This PR migrates the existing Sphinx/MyST documentation under docs/source/ to Fern MDX at docs/fern/, targeting docs.nvidia.com/nemo/run with the NVIDIA global theme. It adds nine converted pages (landing, six guides, FAQs with accordion collapsibles, and a guides index), Fern config/navigation/redirects, shared MDX components, and four GitHub Actions workflows for fern check, doc previews, and publish. Legacy Sphinx sources are left in place for now; API reference from autoapi was not migrated. Preview and publish workflows require the DOCS_FERN_TOKEN org secret.

Test plan

• cd docs/fern && fern check
• Review FAQ accordions and internal links in preview
• Confirm Fern docs CI passes on the PR

Made with Cursor

[svcnemo-autobot]

PR #553 — needs attention
Blocking findings:

1. .github/workflows/fern-docs-preview-comment.yml:42-70 and :110-141 consume PR-controlled artifact metadata/content in a workflow_run job with DOCS_FERN_TOKEN and pull-request write permission. The artifact selects the PR number and supplies the Fern tree. Resolve and validate the PR, head repository, and SHA from trusted workflow-run/API data; never let artifact metadata select the write target, and define a safe boundary before processing untrusted sources with the Fern token.
2. docs/fern/versions/nightly.yml:4-20 publishes an obsolete, incomplete tree. The branch is 143 commits behind main and omits current Quickstart, Architecture, executor guides, and API-reference coverage. Rebase and migrate the current docs before enabling production publish.
3. DCO is ACTION_REQUIRED for both commits.
   Non-blocking findings: docs/fern/components/Authors.tsx:23 imports missing ./devnotes/authors-data; guides/execution.mdx:22 and faqs.mdx:132 document incorrect home-directory names.
   Validation gaps: Fern check passed with one unreported warning, but the token-authenticated preview/comment consumer did not run. The secrets check failed because this stale branch lacks the baseline file, not because it found a secret.