docs: document theme.tabs object config with style, alignment, and placement

[devin-ai-integration]

Summary

Documents the new theme.tabs object configuration introduced in fern-api/fern#12983. The theme.tabs setting now accepts either a string shorthand ("default" | "bubble") or an object with style, alignment, and placement properties.

Changes across 3 files:

• site-level-settings.mdx: Updated the Theme configuration example to use the object form as the recommended approach. Changed theme.tabs type from string → string | object, added new ParamField entries for theme.tabs.style, theme.tabs.alignment, and theme.tabs.placement.
• tabs.mdx: Updated "Tabs placement and styling" to recommend theme.tabs.placement as the primary way to move tabs to the header. Added a full object-form example showing style + alignment + centering.
• changelog/2026-03-30.mdx: New changelog entry for the feature.

Updates since last revision

• Consolidated the Theme configuration section from a two-tab (<Tabs>) component ("String form" / "Object form") into a single code block showing the object form integrated into the full theme example. The string shorthand is still documented in the theme.tabs ParamField description.

Review & Testing Checklist for Human

• Verify anchor links resolve correctly. The cross-references (e.g., #themetabs, #themetabsplacement, #themetabsalignment) and the changelog button link depend on how the docs site generates anchors from ParamField paths with dots. Check that these link to the right sections on the rendered page.
• Confirm layout.tabs-placement is still findable. The tabs.mdx page now leads with theme.tabs.placement instead of layout.tabs-placement. Verify users who search for the old property can still find the correct docs (it's still documented in the Layout section of site-level-settings).
• Spot-check the preview deployment for site-level-settings and tabs to confirm the examples and ParamField entries render as expected.

Notes

• The theme.tabs.placement property takes precedence over layout.tabs-placement when both are set — this precedence note is included in the ParamField description.
• The centering-only-works-with-header constraint is called out in both the site-level-settings page and the tabs page via <Note> components.
• Vale passes clean on all changed files.

Link to Devin session: https://app.devin.ai/sessions/f192b1f289bf4588aad8216445a89c9b

[devin-ai-integration]

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[fern-api]

Requested by: mathieu
Slack thread: View conversation

[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1774894502-document-theme-tabs-hierarchy.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/changelog/2026/3/30
• learn/docs/configuration/site-level-settings
• learn/docs/configuration/tabs

[github-actions]

⚠️ [vale] _{reported by reviewdog 🐶}
[FernStyles.Current] Avoid time-relative terms like 'now' that become outdated