docs: Middleware explicit runtime differences (#83348)
In previous docs versions, while not explicitly stated, it was rather
clearer that middleware has its own runtime.
Also, people who have used bundle-analyzer would for example be familiar
with the 3 bundles, or those who have worked with
`next.config.js`/instrumentation would understand that there's some
different runtimes at play going on there, but those who exclusively
work with middleware might still miss on this detail.
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Co-authored-by: JJ Kasper <jj@jjsweb.site>
