This PR is a last minute addition to Alpha 3 that enables our flexbox CSS as a separate, override stylesheet for our hosted docs. I've disabled it for local development only because my process for testing flexbox support is universal for all our docs (e.g., enable it and work). I've modified the Gruntfile and created a new Sass file to build this override from.
As part of this, I've updated the warning callout to communicate the flexbox enablement to docs visitors, as well as tweaking the bootstrap-grid.scss file to remove the dupe variables.