docs: add important info around global styles (#67778)
## Why?
- We currently don't document why we don't remove styles on navigation.
This was true with Pages Router, but it wasn't necessarily an issue
folks reported on because we reinforced global css to only be available
in `_app.js`. With App Router, we need to be explicit as to why this is
the case.
- We currently don't recommend turning off sort-imports from
linters/formatters since CSS import order matters.
x-ref:
https://github.com/vercel/next.js/issues/58597#issuecomment-2226512480,
https://github.com/vercel/next.js/issues/58597
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Co-authored-by: Delba de Oliveira <32464864+delbaoliveira@users.noreply.github.com>
