Add mdx page example to examples/app-dir-mdx (#47082)

This PR affects the `examples/app-dir-mdx` project.

Currently, only an example of importing a `.mdx` file (`message.mdx`) into the `page.tsx` file is shown.
However, a more common example would be using a `.mdx` file as a page, e.g., in a blog scenario.
I added an example for this, enabled the page extension to allow `.mdx` pages, and changed the body's height, which is a cosmetic change for the newly created blog page.

Author: alexanderbluhm

examples/app-dir-mdx/app/blog/hello-world/page.mdx

@@ -0,0 +1,3 @@
+# Hello World
+
+This is an example blog page with MDX support.

examples/app-dir-mdx/app/globals.css

@@ -83,6 +83,7 @@ html,
 body {
   max-width: 100vw;
   overflow-x: hidden;
+  height: 100%;
 }
 
 body {

examples/app-dir-mdx/next.config.js

@@ -1,5 +1,6 @@
 /** @type {import('next').NextConfig} */
 const nextConfig = {
+  pageExtensions: ['js', 'jsx', 'ts', 'tsx', 'mdx'],
   experimental: {
     appDir: true,
     mdxRs: true,