ExpandableSection - revisit a11y docs and general accessibility

[thatblindgeye]

This is a followup to #3803 which mainly updated the a11y docs to the new template.

We should revisit the general accessibility of the component in both HTML and React libraries as well as updating the a11y docs to match if necessary. This is a general placeholder for a spike discussion/investigation regarding this since it would affect a11y docs if we update the component significantly.