docs: update recommendation for canonical urls to use transformPageData instead

docs/reference/site-config.md

@@ -649,15 +649,16 @@ export default {
 
 ```ts
 export default {
-  transformHead({ page }) {
-    // Skip the 404 page
-    if (page !== '404.md') {
-      const canonicalUrl = `https://example.com/${page}`
+  transformPageData(pageData) {
+    const canonicalUrl = `https://example.com/${pageData.relativePath}`
       .replace(/index\.md$/, '')
       .replace(/\.md$/, '.html')
 
-      return [['link', { rel: 'canonical', href: canonicalUrl }]]
-    }
+    pageData.frontmatter.head ??= []
+    pageData.frontmatter.head.push([
+      'link',
+      { rel: 'canonical', href: canonicalUrl }
+    ])
   }
 }
 ```