misc html/css cleanups

Here's a bunch of HTML / CSS cleanups. Nothing major, but there's a couple of changes to how we render things:

1. The note under the "Featured APIs"-list is no longer oddly oversized compared to the rest of the page.
2. There's a few minor changes to the footer-headings in terms of color, weight and spacing.
3. There's a slight change to the bottom-margin on the 404 page, making it consistent with other pages.

I consider all of those bugfixes, albeit pretty minor ones.