docs: document layout.mobile-toc site-level setting (#5013)

Co-authored-by: fern-api[bot] <115122769+fern-api[bot]@users.noreply.github.com>
Co-authored-by: Devin Logan <devinannlogan@gmail.com>

Author: 3 people

fern/products/docs/pages/changelog/2026-04-20.mdx

@@ -0,0 +1,26 @@
+---
+tags: ["navigation", "configuration", "docs.yml"]
+---
+
+## Mobile table of contents bar
+
+Pages using the [`guide` and `overview` layouts](/learn/docs/configuration/page-level-settings#layout) can now display a sticky table of contents bar below the header on mobile and tablet viewports. The bar shows a scroll progress indicator and the current heading, and expands to the full table of contents when tapped.
+
+<Frame>
+    <video
+      autoPlay
+      muted
+      loop
+    >
+      <source src="./mobile-toc.mp4" type="video/mp4" />
+    </video>
+</Frame>
+
+Enable it by adding `mobile-toc: true` under `layout` in `docs.yml`:
+
+```yaml docs.yml
+layout:
+  mobile-toc: true
+```
+
+<Button intent="none" outlined rightIcon="arrow-right" href="/learn/docs/configuration/site-level-settings#layoutmobile-toc">Read the docs</Button>

fern/products/docs/pages/changelog/mobile-toc.mp4

@@ -621,6 +621,12 @@ layout:
   If set to true, the feedback form won't be rendered. This can be overridden on a per-page basis using the [frontmatter](/learn/docs/configuration/page-level-settings#on-page-feedback).
 </ParamField>
 
+<ParamField path="layout.mobile-toc" type="boolean" required={false} default="false" toc={true}>
+  If set to true, a sticky table of contents bar is shown below the header on mobile and tablet viewports (under the `xl` breakpoint). The bar displays a scroll progress indicator and the current heading, and expands to the full table of contents when tapped.
+
+  Only applies to pages using the [`guide` and `overview` layouts](/learn/docs/configuration/page-level-settings#layout). The bar is hidden when a page has fewer than two headings or when `hide-toc: true` is set in the [page frontmatter](/learn/docs/configuration/page-level-settings#table-of-contents).
+</ParamField>
+
 ## Theme configuration
 
 Customize the visual style of specific UI elements across your documentation site.