Formatting myst .md files

[paugier]

I use mdformat and mdformat-myst to format .md files.

It works so and so but there are a lot of very bad issues in the MyST context, for example no support for directives with ::: (which are converted in standard directives with ```) and contents of directives not formatted.

Therefore, it would be very convenient to be able to format myst .md files with mystmd through a pass

myst -> ATS -> (formatted) myst