* Add agenda widget with accordion
* Add AgendaItem, README, and do some cleanup
* Handle PR Feedback: Rename AgendaAccordion to AgendaGroup, accept an href prop for a non-accordion state. Accept onClick or href for AgendaItem, a11y fixes, fix some typos, update examples & README.
* Address PR Feedback: Rename AgendaGroup to AgenaHeader, use a direct link for href mode, use IconButton instead of Button & Dashicon, pull out link rendering to its own method. Use <Link /> for app links.
* Combine box-shadow focus/hover rule and add transparent background
* Make `count` optional
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