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.
