Overhaul of the whole download page layout

[rxaviers]

This issue is a follow-up of Scott's comment on #178 (comment):

I'd suggest an overhaul of the whole page layout. We can add a CSS Components section and rename Components to JS Components. Instead of three columns, we should have the headers full width with their descriptions. The component names and descriptions don't need so much visual spacing, they can just be thrown together with the name being bold and followed by a colon. The Theme section can probably just become part of the CSS Components section. We can probably put JS Components and CSS Components side by side in two columns to prevent the page from becoming super long.

I'm not sure what we'd do about older versions though.