Update App.js

[cosmic-snow]

I've added id attributes to <h2> titles for linking with fragment links, e.g. #performance-benchmarks.

Note: I'm really unfamiliar with React, so this might be entirely the wrong way to do it. In that case, just close this, but you get the idea.

The main goal here was to be able to directly link to 'Model Explorer', for the people who have download problems (see the other PR).