[docs] Improve accessibility in the component examples

[tuukkao]

• Added labels to the unlabeled IconButtons.
• Added aria-expanded to one of the Card example buttons to signify that the button in question expands the contents of the card.

The labels are intended to convey the meaning of the button rather than to describe the icons themselves. Please let me know if some of these labels seem incorrect.

Closes #6956.

• PR has tests / docs demo, and is linted. (I got 67 linting errors in the unmodified next branch alone. Is this expected to be broken in next?)
• Commit and PR titles begin with [ComponentName], and are in imperative form: "[Component] Fix leaky abstraction".
• Description explains the issue / use-case resolved, and auto-closes the related issue(s) (http://tr.im/vFqem).

[oliviertassinari]

@tuukkao Thank you :)