Resolve slash-less content links against the site root

A dozen content links are written like [Setup](learn/setup). The Pages
Router's Link resolved those against the origin, so they rendered as
/learn/setup; the App Router's Link leaves them relative to the current
page (/learn/learn/setup -> 404). Restore the old behavior.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: gaearon

src/components/MDX/Link.tsx

@@ -35,18 +35,21 @@ function MDXLink({
   if (!href) {
     return <a href={href} className={className} {...props} />;
   }
+  if (href.startsWith('https://')) {
+    return (
+      <ExternalLink href={href} className={cn(classes, className)} {...props}>
+        {modifiedChildren}
+      </ExternalLink>
+    );
+  }
+  // Some content links to other pages without a leading slash, e.g.
+  // [Setup](learn/setup). The Pages Router resolved those against the site
+  // root; the App Router's Link would resolve them against the current page.
+  const internalHref = /^[a-z]/i.test(href) ? '/' + href : href;
   return (
-    <>
-      {href.startsWith('https://') ? (
-        <ExternalLink href={href} className={cn(classes, className)} {...props}>
-          {modifiedChildren}
-        </ExternalLink>
-      ) : (
-        <Link href={href} className={cn(classes, className)} {...props}>
-          {modifiedChildren}
-        </Link>
-      )}
-    </>
+    <Link href={internalHref} className={cn(classes, className)} {...props}>
+      {modifiedChildren}
+    </Link>
   );
 }