Document screenshot differences for manual readers
Another area we need to document somewhere in the introduction of the manual: that differences in screenshots of dialogs can be expected due to Gnome on Linux having a headerbar with buttons, while e.g. on Windows the buttons are on the bottom of the dialog.
See e.g. the screenshot in !145 (merged)
Also document other possible differences due to themes and platforms (same theme can look different depending on platform used).