add custom frontmatter (#1480)

Author: ethanpalm

organize/pages.mdx

@@ -10,10 +10,13 @@ Each page is an MDX file, which combines Markdown content with React components
 
 Every page starts with frontmatter, the YAML metadata enclosed by `---` at the beginning of a file. This metadata defines how your page appears and behaves.
 
-Use metadata to control:
+Use frontmatter to control:
 
 - Page titles and descriptions
 - Sidebar titles, icons, and tags
+- Page layouts
+- SEO meta tags
+- Custom metadata
 
 <ResponseField name="title" type="string" required>
   The title of your page that appears in navigation and browser tabs.
@@ -47,6 +50,10 @@ Use metadata to control:
   A tag that appears next to your page title in the sidebar.
 </ResponseField>
 
+<ResponseField name="<custom>" type="string">
+  Any valid YAML frontmatter. For example, `product: "API"` or `version: "1.0.0"`.
+</ResponseField>
+
 ```yaml Example YAML frontmatter wrap
 ---
 title: "About frontmatter"