Clarify custom frontmatter fields are not added to HTML head

- Fix og:sitename to og:site_name in frontmatter example
- Add note to SEO metadata page that custom fields aren't supported
- Add note to frontmatter page SEO section with same clarification
- Link to custom JavaScript docs for analytics tracking workaround

Co-Authored-By: Devin Logan <[email protected]>

Author: devin-ai-integration[bot]

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

@@ -16,7 +16,7 @@ subtitle: Set titles, add meta descriptions, and more
 slug: frontmatter
 description: Use frontmatter to set the page title, subtitle, slug, meta description, its OpenGraph image, and a URL to suggest edits.
 keywords: frontmatter, seo, customization, metadata
-og:sitename: Your Company Inc.
+og:site_name: Your Company Inc.
 og:title: SEO Metadata Title
 ---
 ```
@@ -253,6 +253,8 @@ Currently, relative paths are _not_ supported for this field.
  [Use the metadata field in the `docs.yml` file](/learn/docs/configuration/site-level-settings#seo-metadata-configuration).
 </Note>
 
+Only the documented SEO fields below are converted into HTML meta tags. Custom frontmatter fields are not added to the page head. For custom analytics tracking, see [custom JavaScript](/learn/docs/customization/custom-css-js).
+
 <Markdown src="/snippets/seo-metadata-page.mdx" />
 
 ## Changelog tags

fern/products/docs/pages/seo/metadata.mdx

@@ -61,4 +61,8 @@ Set default SEO metadata for your entire documentation site in [`docs.yml`](/doc
 
 Configure SEO metadata in your page's [frontmatter](/docs/configuration/page-level-settings) to control how individual pages appear in search results and social media shares. These settings override site-wide settings.
 
+<Info>
+Only the documented SEO fields below are recognized. Custom frontmatter fields are not added to the HTML head. For custom analytics tracking, use [custom JavaScript](/learn/docs/customization/custom-css-js).
+</Info>
+
 <Markdown src="/snippets/seo-metadata-page.mdx" />