FLUID-5516: Updated build site to match new design.

@jobara @acheetham - I've updated the build site to match the new designs. Can you check to make sure I got all the right demos and that the links all go to the right places?
I should have style the ToC container to have the 'max-width: 66rem;' style like the rest of the content, and the actual "Table of Contents" heading is too big; sorry!