docs: add layout.api-reference-expand-properties

[devin-ai-integration]

Fern docs sites now support an opt-in docs.yml setting, layout.api-reference-expand-properties, that renders the first level of nested object and union fields in the API Reference expanded on page load instead of hiding them behind a "Show N properties" button. Deeper nesting stays collapsed, so pages don't balloon for deeply nested schemas. Customers asked for this because readers previously had to click through to see request and response properties; this documents the new option so they can turn it on.

Implements docs for: feat(docs): add layout.api-reference-expand-properties to docs.yml (fern-api/fern#17677) — fern-api/fern#17677 and feat(docs): layout.api-reference-expand-properties opens first-level nested fields by default (fern-api/fern-platform#14522) — https://github.com/fern-api/fern-platform/pull/14522

Pages changed

• fern/products/docs/pages/navigation/site-level-settings.mdx — new layout.api-reference-expand-properties ParamField (boolean, default false) after layout.api-reference-layout, plus the key in the example layout block.
• fern/products/docs/pages/api-references/customize-api-ref.mdx — short paragraph and YAML snippet under "Nested object fields", linking to the reference entry.
• fern/products/docs/pages/changelog/2026-09-11.mdx — new Docs changelog entry (api-reference, docs.yml).

Summary

Validation: pre-commit run --files <changed> (Vale: no findings on added lines; remaining errors are pre-existing on untouched lines) and fern check --warnings (0 errors).

Link to Devin session: https://app.devin.ai/sessions/1a851d99271b4cb6adc3b967e75aa3cd
Open in Devin Desktop: https://app.devin.ai/desktop/session/1a851d99271b4cb6adc3b967e75aa3cd?variant=devin

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring

[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1789146261-api-reference-expand-properties.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/api-references/customize-api-reference-layout
• learn/docs/changelog/2026/9/11
• learn/docs/configuration/site-level-settings