doc: clarify rewrites execution order (#93765)
Rework the execution order of rewrites.
- before and after Files execution is now clearer - when a source
matches and how the destination resolves
- explicit `dynamic routes` step (similar to Proxy's,
https://nextjs.org/docs/app/api-reference/file-conventions/proxy#execution-order)
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Co-authored-by: JJ Kasper <jj@jjsweb.site>
