Various improvements

- more whitespace around promo copy on index page
- rearranged some of the feature elements 

I tried to reorder some things by what I thought is more significant about boot.

I'd also like to reword the last two points to be more about positive features than potentially negative properties of other build tools. Thinking of "Tasks are not coupled" & "Artifacts can never be stale" here. Haven't had the right idea just yet though.