Update docu for Stanton-DJC-4: Auto DJ

[xxxmicrobexxx]

Added AutoDJ toggle and AutoDJ Xfade to bottom of mixer section

https://github.com/mixxxdj/mixxx/pull/13800

[ronso0]

Thank you! (the linkcheck fail is unrelated)

What do you think about adding an overview image of the controller? I found this in the official manual, looks pretty clean IMO. I optimized it for the manual (transparent bg) and can provide a commit for cherry-picking if you agree.

[ronso0]

Btw, I just noticed that there seems to be a clone, the OMNITRONIC DDC-2000