Add 40px anchor margin to make room for bootstrap header

Currently on [the website](http://probcomp.csail.mit.edu/bayesdb/#Overview), if you click any of the nav links at the top you are brought down to the anchor, but because of the nav height, it obstructs part of the heading.

This change adds a 40px margin to make room for the headings. Solution from [StackOverflow](http://stackoverflow.com/questions/5070751/anchor-links-and-margins).