docs: document heading-based steps syntax and tocDepth prop

[devin-ai-integration]

Summary

Documents three previously undocumented aspects of the <Steps> component:

1. Markdown heading syntax: Users can define steps using ## or ### headings inside <Steps> instead of explicit <Step> components. This syntax already works but was not documented.
2. TOC depth from heading level: When using toc={true} with heading-based steps, the heading level now determines the depth at which steps appear in the table of contents (e.g., ### headings nest under ## headings). This behavior is being added in the companion PR fern-api/fern-platform#7157.
3. tocDepth prop: New property on <Steps> that controls the depth at which steps appear in the table of contents. Automatically set when using heading-based syntax; can be set manually when using explicit <Step> components.

Changes are scoped to a single file: fern/products/docs/pages/component-library/default-components/steps.mdx.

Updates since last revision

• Replaced all uses of the "TOC" abbreviation with "table of contents" to pass vale lint checks.
• Added tocDepth prop documentation as a new <ParamField> under <Steps> properties, with a code example showing manual usage with explicit <Step> components.
• Added expanded description to the toc prop explaining how heading levels affect table of contents nesting.

Review & Testing Checklist for Human

• Verify tocDepth can actually be set by users on <Steps> — the prop is documented as user-facing, but in fern-platform it's primarily set internally by the rehype-steps plugin. Confirm that <Steps toc={true} tocDepth={2}> with explicit <Step> children actually produces depth-2 entries in the table of contents. The toc.ts code reads the attribute regardless of source, so this should work, but it hasn't been tested end-to-end.
• Docs claim tocDepth accepts values 1, 2, or 3 — the rehype-steps plugin caps at 3, but toc.ts does a raw parseInt with no upper/lower bound validation. A user setting tocDepth={5} or tocDepth={0} would pass through unclamped. Decide if the docs should mention this or if the platform code should add validation.
• Coordinate merge order with fern-platform#7157 — the tocDepth behavior documented here depends on that PR being merged first. Merging docs first would document behavior that doesn't exist yet.
• No live rendering example for heading syntax — unlike other variants ("Steps with titles", etc.), the new "Steps with markdown headings" section only has code blocks, not a highlight-frame live demo. Decide if a live example should be added for consistency.

Suggested test plan: After merging fern-platform#7157, check the preview deployment and verify (1) the code examples render as syntax-highlighted blocks (not live headings), (2) the tocDepth ParamField renders correctly with its nested code example, and (3) the page table of contents looks sensible.

Notes

• Link to Devin run
• Requested by: paarth@buildwithfern.com

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[github-actions]

📝 [vale] _{reported by reviewdog 🐶}
[FernStyles.Acronyms] 'TOC' has no definition.

[github-actions]

📝 [vale] _{reported by reviewdog 🐶}
[FernStyles.Acronyms] 'TOC' has no definition.

[github-actions]

🌿 Preview your docs: https://fern-preview-298206d8-c3f0-48e8-8681-79d4d6a54ad1.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/changelog/2026/2/17
• learn/docs/writing-content/components/steps

[github-actions]

🚫 [vale] _{reported by reviewdog 🐶}
[Microsoft.Contractions] Use 'they're' instead of 'they are'.