[docs] Update docs related to using Next.js's Link with Material UI

[mj12albert]

The API for <Link/> is very different for Pages Router and App Router:

• App Router Link: https://nextjs.org/docs/app/api-reference/components/link
• Pages Router Link: https://nextjs.org/docs/pages/api-reference/components/link

The docs on routing here should be updated

TODO:

• 1. Remove legacyBehavior={true}
  • [docs] Fix element.ref accessing warning on docs #45155
  • [docs][nextjs] Migrate from deprecated "legacyBehavior" prop #44871
• 2. NextLinkComposed doesn't seem to make any sense anymore/
  • Update https://mui.com/material-ui/integrations/routing/#next-js-pages-router to remove it
  • Stop exporting it from the components