Docs: (subjectively) nicer looking accordion example (#31137)

Docs: (subjectively) nicer looking accordion example (#31137)

* Docs: (subjectively) nicer looking accordion example

the use of `.btn-link` currently just looks a bit odd, subjectively (as in v5 we now force the underline to always be there, while in v4 at least it was only on hover/focus).
`.btn-light` seems more fitting

* Add extra classes for nicer overall look

as suggested by @ysds

* Tweak accessibility note for accordions and optional keyboard interactions