feat: add rehype-external-links for external link security

app/posts/[slug]/page.tsx

@@ -58,8 +58,12 @@ export default async function PostPage({ params }: { params: Promise<{ slug: str
                 options={{
                   mdxOptions: {
                     remarkPlugins: [smartypants, remarkMath, remarkGfm],
-rehypePlugins: [
+                    rehypePlugins: [
                       rehypeSlug,
+                      [rehypeExternalLinks, {
+                        target: '_blank',
+                        rel: ['nofollow', 'noopener', 'noreferrer'],
+                      }],
                       rehypeAutolinkHeadings,
                       [rehypePrettyCode, { theme: 'github-dark' }],
                       rehypeKatex,