Now that sphinx-gallery supports sub-folder sorting within a gallery, let's re-organize examples / tutorials to make sure that the most important folders come first.

Do folks have an opinion about which folders should be at the top? If not I'll take a stab at a PR that seems to make the most sense to me.

Also I don't think that tutorials really need reordering. BUT, we could remove the 01_ and 02_ etc if wanted, since those were used for ordering below. Thoughts?