A long overdue rewrite of the accessibility section - instead of the few snippets of strangely superficial and out-of-context advice (skip links, use correct heading levels), this tries to answer some of the fundamental questions about "is Bootstrap accessible", with emphasis on the fact that the final result will depend in large part on what BS is applied to/on (since BS relies on the markup etc authored by developers). This also sets out our ambition to have things work for keyboard and assistive tech users, and that we strive to make all our examples etc accessible and semantic.