docs: add beta banner frontmatter property documentation

Add documentation for the new beta frontmatter property that displays a yellow warning banner on pages to indicate beta/work-in-progress content.

Co-Authored-By: Deep Singhvi <[email protected]>

Author: devin-ai-integration[bot]

fern/products/docs/pages/customization/frontmatter.mdx

@@ -213,6 +213,29 @@ hide-feedback: true
   <img src="./on-page-feedback.png" alt="Leave feedback feature" />
 </Frame>
 
+## Beta banner
+<ParamField path="beta" type="boolean | string" required={false} default={false}>
+  Displays a yellow warning banner at the top of the page to indicate that the content is in beta or still being worked on. Set to `true` for a default message, or provide a custom message string.
+</ParamField>
+
+<CodeBlock title="Example beta banner with default message">
+```mdx
+---
+title: New Feature Documentation
+beta: true
+---
+```
+</CodeBlock>
+
+<CodeBlock title="Example beta banner with custom message">
+```mdx
+---
+title: Experimental API
+beta: This API is experimental and may change without notice.
+---
+```
+</CodeBlock>
+
 ## Page logo
 <ParamField path="logo" type="object" required={false}>
   Override the site-wide logo for a page. Specify different logos for light and dark modes using absolute URLs.