I'm a bit surprised to find the MediaWiki project absent on this page. I assume there is a good reason for this; something like complexity, lack of mentoring or development documentation, or something else entirely.
But that doesn't mean that it shouldn't be mentioned, at the very least as an intermediate option, perhaps at the bottom of the list. We have a phabricator project good first task with things that new developers can do.
It also does not help that the page links to New Developers#Choose a software project on the same page, which does not list MediaWiki, but the next item, "Set up your development environment" is irrelevant for any of the mentioned projects! It focuses solely on MediaWiki.