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
This might be an instance of teaching music theory as opposed to showing how to use LilyPond, Jan. I've been assuming that our audience is able to read music well enough to want to engrave their own, so the extra material would not really be needed.