Nested accordion example with bordered items

Add the "Nested accordion example with bordered items" component (reui/c-accordion-8, from ReUI) to this project.

1. Run: npx get-ui-cli add reui/c-accordion-8 --yes
2. Read the "Use it" snippet it prints, then import the component and render it where it fits.
3. Don't rewrite the installed file. Adapt it through its props and className, so `npx get-ui-cli update` can still merge upstream fixes.
4. Run: npx get-ui-cli doctor, and fix anything it reports.