Update contentlayer demo

Author: pomber

examples/contentlayer/README.md

@@ -1,3 +1,3 @@
 # Next.js + Contentlayer + Code Hike
 
-See [this guide](https://codehike.org/docs/installation/contentlayer).
+See [this guide](https://codehike.org/docs/installation/contentlayer) or try it on [Stackblitz](https://stackblitz.com/github/code-hike/codehike/tree/main/examples/contentlayer).

examples/contentlayer/posts/one.mdx

@@ -0,0 +1,10 @@
+---
+title: Post one
+---
+
+This is the first post.
+
+```python hello.py
+# mark[16:24]
+print("This is Code Hike")
+```

examples/contentlayer/posts/post-one.mdx

@@ -0,0 +1,9 @@
+---
+title: Post two
+---
+
+This is the second post.
+
+```js foo.js
+const x = 2
+```