markdown output

hello,

I think that having doxygen supporting the markdown output would be useful. Right now there are other tools that accomplish something similar using the xml output.
For example: [moxygen](https://github.com/sourcey/moxygen) and [doxygen2md](https://github.com/pferdinand/doxygen2md)

The existence of these project means that I am not the only one who think that this would be useful ✌️ 