Keep the structure a bit more clean.

We want to show code examples in different languages, and the
sphinx-tabs extension allows us to do that in a way that is easy to do,
and looks good.

The old platform-demos tutorials in gnome-devel-docs should be coalesced
into a single set of beginners tutorial and overview of GTK (and
possibly other libraries). Instead of having wildly different content,
we should merge everything into a singular text, with a consistent
voice, and have code examples in different languages.

The beginners tutorial for Python is a fairly good base for that;
additionally, we have HowDoI content for some widgets that should find a
new home in the tutorial.