Update GTK on Windows link: use gtk.org, not wiki

Update GTK on Windows link: use gtk.org, not wiki

The old wiki page has a couple issues:
* It's out-of-date, not having any notes for GTK4 specifically,
  and it doesn't link to `gvsbuild`, which (I believe) is
  the current official (and best) way to build GTK with MSVC.
* It's "locked", so it's harder for contributors to update. This
  is OK, except for one spicy detail:
* It's not clear how to contribute/report issues on the locked wiki
  pages, so out-of-date information falls off the radar.

Regardless :) the gtk.org GTK for Windows docs are a much better
springboard, in my opinion.