ARIA / screenreader support for collapse.js

Added handling of aria-expanded=true/false to collapse.js, updated
documentation to include advice on making expand/collapse controls
accessible, updated examples and javascript documentation to use
aria-expanded and aria-controls (when targetting single collapsible
element, using ID rather than class selector)
Maybe you could handle these lines here [like this](https://github.com/twbs/bootstrap/pull/14153/files#diff-440894180ffcb133a60f858a3a4f1008R168)?