Keyboard accessible accordions

[Malvoz]

From Leaflet/Leaflet#8006 (comment):

Misc / General site issues

• Accordions in the Leaflet API reference are not keyboard accessible.
  Replace custom <div> accordion (inherited.hbs, reference.js) with <details>? Or handle it via tabindex="0", role="button", aria-expanded, etc.?
  This is (also) a Leafdoc accordion issue.

Support data for <details> and <summary>: https://caniuse.com/details.

I'm not sure if/when I'll get to this, hopefully others can contribute.

[Malvoz]

Maybe this should wait until "auto-expand <details> on find-in-page and fragment navigations" is widely supported.

Filed Fyrd/caniuse#6472.