docs/zh_CN: restructure how-to.rst patch submission workflow

Split "导出补丁和制作封面" into separate "导出补丁" and
"为补丁集制作封面" sections, and document the single-patch
(git format-patch -1) and multi-patch (-N) flows side by side
so new contributors do not have to infer one from the other.

Replace the invalid "git am --amend" invocations with "git commit
--amend" in both the checkpatch fix-up and the iteration sections.

Expand the iteration section with a worked v2 example showing where the
changelog goes relative to the --- separator, and describe how v3/v4
changelogs stack newest-on-top.

Finally, add a note reminding submitters to carry Reviewed-by tags from
reviewers into the next revision, placed below Signed-off-by.

Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Dongliang Mu <dzm91@hust.edu.cn>
Reviewed-by: Yanteng Si <si.yanteng@linux.dev>
Signed-off-by: Alex Shi <alexs@kernel.org>

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