should work

svelte.config.js

@@ -1,11 +1,18 @@
 import adapter from '@sveltejs/adapter-auto';
 import { vitePreprocess } from '@sveltejs/kit/vite';
+import { mdsvex } from 'mdsvex';
 
 /** @type {import('@sveltejs/kit').Config} */
 const config = {
 	// Consult https://kit.svelte.dev/docs/integrations#preprocessors
 	// for more information about preprocessors
-	preprocess: vitePreprocess(),
+	extensions: ['.svelte', '.md'],
+	preprocess: [
+		vitePreprocess(),
+		mdsvex({
+			extensions: ['.md']
+		})
+	],
 
 	kit: {
 		adapter: adapter()