docs: move developers article to main website
This isn't content that relates directly to a specific release of Mesa, and it's also quite out-of-date. Let's move it to the main mesa website instead, where we have an updated version.
While we're at it, let's also retire thanks.html, which is also horribly out-of-date.
See mesa3d.org!167 (merged) for the mesa3d.org side of this.