New hero for builds page

[locks]

The builds page has an assortment of problems, let's try to tackle them progressively.

The current call to action uses the same styling as the warning for when you are viewing an older version of the guides, which is an undesirable association.
We should introduce a hero 100vh-above-the-fold type of thing to with the current version, a link to the release blog, and a link to the respective Guides.
It would also be nice to transform the outdated table of contents at the left into an horizontal sub-menu. Feels free to keep index, all three channels, and drop "prev. releases".

cc @MelSumner

[joukevandermaas]

I did a quick mockup of this: http://awesan.com/emberjs-release-mockup/

I will try to update the real thing to look something like that if you agree. Of course all the explanation etc. that's on the current build page won't be lost.