[Style guide] Added property (#20693)

Author: kodster28

src/content/docs/style-guide/frontmatter/custom-properties.mdx

@@ -15,3 +15,4 @@ We have added specific custom [frontmatter](/style-guide/frontmatter/) to meet s
 - `products` <Type text="Array<String>" /> <MetaInfo text="optional" />: The names of related products, which show on some grids for Examples, [Tutorials](/style-guide/documentation-content-strategy/content-types/tutorial/), and [Reference Architectures](/style-guide/documentation-content-strategy/content-types/reference-architecture/)
 - `tags` <Type text="Array<String>" /> <MetaInfo text="optional" />: A group of related keywords relating to the purpose of the page.
 - `updated` <Type text="Date" /> <MetaInfo text="optional" />: This is used to automatically add the [LastReviewed component](/style-guide/components/last-reviewed/).
+- `noindex` <Type text="Boolean" /> <MetaInfo text="optional" />: If true, this property adds a `noindex` declaration to the page, which will tell internal / external search crawlers to ignore this page. Helpful for pages that are historically accurate, but no longer recommended, such as [Workers Sites](/workers/configuration/sites/).