Revert "Keyboard accessibility in documentation"

This reverts commit a809daf2841f65ffac5030b2c6ce6aea60dd6233.
Leave tabindex=-1 alone

It seems that, as misguided as I personally think it is, ARIA does
indeed want dropdown menus not to behave as regular keyboard users
expect them, killing TAB/SHIFT+TAB in favor of cursor keys. Fair
enough, the issue I have is then with ARIA, not with bootstrap. I would
sugges thought that if you really do want to follow ARIA consistently,
*any* navigation bar should also become an ARIA menu, further making it
impossible for keyboard users to use TAB/SHIFT+TAB to navigate through
it, and that these changes should be reflected in the bootstrap
examples as well...