Docs: Add "Sort by Server Channel" to mixer view

https://github.com/jamulussoftware/jamulus/issues/3417 / https://github.com/jamulussoftware/jamulus/pull/3418 added a new option to the main window View menu - Sort by Server Channel.

The documentation and screenshots need updating to match.