update ping1d firmware specifier to latest version (3.28 -> 3.29)
add separated instructions for original Ping Sonar vs new Ping2
a button switcher might make more sense (especially once there's also Ping360 support), but I've kept it simple for now
improve the README for people who want to build the docs
NOTE: I've changed the Linux flashing command slightly, because I believe it was wrong before (using .\ like Windows), but I haven't confirmed that the new form is actually correct
Updates: