Update Quick Start Guide

Update by convert-ly to 2.19.7
Shifted emphasis away from Windows to be more general
Updated links
Re-arranged some material
Many minor changes of punctuation and spelling
It seemed to me that notes are "accidental" only in relation to an established key signature. The need to specify the degree of a pitch only comes in when it alters the expected one, if you see what I mean. That's why I moved the key signature section before accidentals, moving from the general case to the exceptions.