examples: update README.md in `mdx` example (#73454)
## Description
[`mdx`](https://github.com/vercel/next.js/tree/canary/examples/mdx)
example
[README.md](https://github.com/vercel/next.js/blob/canary/examples/app-dir-mdx/README.md)
is inaccurate.
For instance, API routes is described even though it does not exist.
In conclusion, I rewrote the README.md following [example
guidelines](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md).
CC: @samcx
### Adding or Updating Examples
- [x] The "examples guidelines" are followed from our contributing doc
https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md
- [x] Make sure the linting passes by running `pnpm build && pnpm lint`.
See
https://github.com/vercel/next.js/blob/canary/contributing/repository/linting.md
