feat: support markdown (md/mdx) files on disk

[joelhooks]

Describe the feature you'd like to request

primarily for legacy purposes, but if we add a markdown file it should "just work" as expected

Describe the solution you'd like to see

does the app router as setup just handle this?

Additional information

considering the use case of migrating an existing "blog" to Course Builder

👨‍👧‍👦 Contributing

• [x] 🙋‍♂️ Yes, I'd be down to file a PR implementing this feature!