docs: Use phosphor icons import perf optimization (#80811)
Fixes: https://github.com/vercel/next.js/issues/80800
`react-icons` only supports the pattern we show through
`react-icons/all-files`, which hasn't been updated in the past 5 years.
`react-icons` does group icons by sets, so the recommendation about
sticking to a set is still valid.
I chose `@phosphor-icons/react`, since they also have a section on why
importing only the icon you need is a good idea.
