add platform-agnostic installation links; clean up Hello World and Next Steps
add platform-agnostic installation links; clean up Hello World and Next Steps

KevinFrans3/piston,PistonDevelopers/piston,axelmagn/piston,PistonDevelopers/piston,caldwell/piston,bvssvni/piston,getynge/piston,placrosse/piston,bvssvni/piston,tempbottle/piston,oldmanmike/piston,Potpourri/piston,mitchmindtree/piston