Fix horizontal scrollbars caused by nav bar

[davepagurek]

Resolves https://github.com/processing/p5.js-website/issues/289

Turns out it was because it was using 100vw, which does not subtract the width of the vertical scroll bar, causing a slight overflow the size of the scroll bar width. Using 100% seems to fix it. I also replaced a hardcoded 200px with the variable for the sidebar offset.