Update GNOME Builder section
Since the App Development page was written, GNOME 3.28 has come out and should be the standard for flatpak runtime as well as the stable 3.28 GNOME Builder application used (instead of some random nightly).
Also an intro is needed on the initial Apps/Gnome.html page. Mention that GNOME is the default and what kinds of tools to use to develop an app for GNOME.