programming-guidelines: Rework introduction to cover the GNOME ecosystem

These guidelines are useful enough to cover development of any
application which uses GNOME technologies — not just development of core
GNOME applications. Rework the introduction to fit that a little better.

Also cheekily add myself as an author in anticipation of further
commits.

https://bugzilla.gnome.org/show_bug.cgi?id=376123