chore: add mdc example

examples/with-markdown/astro.config.mjs

@@ -0,0 +1,5 @@
+export default {
+  markdownOptions: {
+    components: './src/components/index.tsx'
+  }
+}

examples/with-markdown/src/components/index.tsx

@@ -0,0 +1 @@
+export { default as PreactCounter } from './PreactCounter.tsx';

examples/with-markdown/src/pages/markdown-plus-components.mdc

@@ -0,0 +1,12 @@
+---
+title: Some Markdown Page
+layout: ../layouts/main.astro
+---
+
+# Paragraph
+
+text here.
+
+<PreactCounter client:visible />
+
+Hello world!