Add documentation for mdDown

[lilouartz]

Subject

Breakpoints

Description

I noticed that I am using an undocumented feature. I cannot find anywhere in the documentation ${breakpoint}Down mentioned, but it is working, and I have not a clue where I picked it up from.

[anubra266]

@lilouartz Feel free to open a PR.
It'll be somewhere around here
https://panda-css.com/docs/concepts/responsive-design#targeting-a-breakpoint-range