Hi
I play the accordion and occasionally arrange some pieces with lilypond.
The systems of the buttons on the right hand (button accordion) and free bass (left hand) are organised chromatic in three rows. Therefore the fingerings for scales and chords can be shifted by a minor third. Depending on the instrument there are also additional repeated rows - medium-sized button-accordions have five rows on the right hand so that a scale can be played with the same fingering in any key.
Typically the layout (and also the "standard" or "stradella" bass system) is illustrated with schematic diagrams like a diagonal grid of circles with numbers or note names in it.