Document Jukebox.AdminOnly

Apparently, a new Jukebox.AdminOnly config bool was recently added, and it defaults to true. This needs to be documented on `/docs/usage/jukebox/` and `/docs/usage/configuration-options/`. So people are not left lost as to why jukebox doesn't work. (perhaps if the website src lived in the same repo as the code, the keeping up to date of docs could be more smoother)