Shortlinks for tutorials

I created 2 links that server admins can put into MOTD or similar welcome messages to let players know about our tutorials. It should be better than services like bit.ly because it looks more official and we can change the link after it has been created in case the pages get renamed again.

The second commit puts the redirect pages into a subfolder as suggested by @detrate in case we ever decide to add more. The downside is that using the short version like xonotic.org/tut takes 2 redirects (first the alias, then the real one). We could also use a short folder name like r to get xonotic.org/r/tut but imho the shorter the better even though the redirecting takes slightly longer.