Reverts vercel/next.js#40016.
`pageExtensions` is not frequently used – as is mostly a workaround
until the Layouts RFC lands. Thus, it doesn't make sense to place a note
on the Middleware page and expect readers to need to connect the dots
and understand `pageExtensions`.
Further, the example docs linked could be more specific about Middleware
to prevent this confusion. I will also make that change here.
Co-authored-by: Balázs Orbán <info@balazsorban.com>
4f793fd7fa