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)
Perhaps we could make this a regular paragraph in the docs instead of a callout? We probably have too many callouts already.
Also, not sure this belongs under the "Via data attributes" section.