Add "Brand guidelines" page to documentation

So far all it has is color values. I'll add more content as I find it
from the various places where this has been documented before.
Welcome page, and update brand guideleines page

Removed the "Welcome to Bootstrap" page, and added a "Brand GUidelines"
page. So far the only thing on it is the color variables, but it can be
expanded with more as its codified.