docs: submitting-patches: Discuss interleaved replies

Top-posting has been strongly discouraged in Linux development, but this
was actually not written anywhere in the common documentation about
sending patches and replying to reviews. Add a section about trimming
and interleaved replies.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230511184131.gonna.399-kees@kernel.org