We moved from the Ruby compiler to sassc in
commit 67953e9cfb, so this copies across
the updated info about building from GTK+ 4.
Also, explain the purpose of parse-sass.sh, since while that is not
mentioned in GTK+ 4 – and perhaps does not need to be, thanks to Meson –
we are still on Autotools here, and rebuilding the entirety of GTK+ 3 if
you only edited the CSS is a lot of waiting for no good reason.
This subdirectory gets in the way when integrating the inspector
build more fully with GTK+, and does not really add anything.
Just move everything one level up.