Add mdformat formatter

[Libadoxon]

Checklist (adding a new tool):

• The tool was added to the README
• I do not wish to write a test, but I can confirm that it works on my machine OR I have written corresponding tests for the tools I added. (don't worry if you didn't write a test, it's just nice to see the green check mark)

[xiaoshihou514]

Thanks for the pr!