check-patches: enforce trailer list

With the new Changelog-*: trailers it's important that trailers are not
misspelled accidentally and to cut down on trailer clutter. Add an
explicit list of accepted trailers with their preferred ordering and
some explanation where warranted to CONTRIBUTING. Enforce the list in
CI.

Signed-off-by: Bence Ferdinandy <bence@ferdinandy.com>
Reviewed-by: Moritz Poldrack <moritz@poldrack.dev>
Acked-by: Robin Jarry <robin@jarry.cc>

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