Updates needed to "Developing for GNOME" section

Going through this section, some updates are needed:

Setting up flatpak: Update flathub org.gnome.Platform//3.30 to flathub org.gnome.Platform//3.32
Pic at the bottom of GBuilder page is super old and we should update it with actual screenshots of calls. Possibly other bits of this page need updating as well.

Edited Sep 16, 2019 by David Boddie

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