More docs improvements

Those are the UI tweaks I had discussed with @molily. In the end I didn't go for a "Chaplin Documentation" heading in the sidebar, since it takes up so much space I would rather use to see more of the API Reference without scrolling. Maybe this is only my 11" screen bias. Feel free to complain!

See also my comment in 71a2844 about the "@foo"/"this.foo" question.