Fix some links

[stevekrouse]

I was going through your site and noticed that a lot of the links were broken and I said "I wish there was a bot to fix this" and my cofounder @tmcw heard me and said, "oh I built something that does that", so we pointed his https://github.com/tmcw/notfoundbot at your repo and got some decent fixes.

I was hoping it would fix the links on your index.html page because that's the one I was going through, but I wanted to send over the PR regardless