Add heading anchors for every title in a doc page

[jvican]

I am loving this theme! You can see how I'm using it in https://scalacenter.github.io/bloop/. Thanks for the work that has gone into this :smile:

Now, I'm trying to modify the theme to add heading anchors for every title in a doc page. I'd like to know if you've already thought about this use case and you know a way to have it without implementing it from scratch.