SSH: add some tmux documentation

SSH should only be used for debugging purposes but it can also sometimes be used to run some scripts (migration and etc..). We should document tmux (available on all instances) so that users can start their scripts without having to worry about having their connections breaking.

Basic usage like attach (or create), maybe split and more importantly, detach. Maybe there are some good articles out there we could link to for a more advanced usage of tmux, if needed.