docs: add Ctrl-J as newline alternative in keyboard shortcuts

[Eric-Song-Nop]

Related Issue

#256

Problem

在pr #9 中kimi-code保证了ctrl+j快捷键在所有终端中都可以一致的进行换行并在 /help 中添加了说明，但是作为更一致的快捷键并没有在reference等其他文档站中进行说明导致了诸如 #256 的困惑。

What changed

Updated keyboard shortcut references in both English and Chinese documentation to mention "Ctrl-J" alongside "Shift-Enter" for inserting a newline.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works.
• Ran "gen-changesets" skill, or this PR needs no changeset.
• Ran "gen-docs" skill, or this PR needs no doc update.

[changeset-bot]

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Latest commit: 00cf8b0

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[Eric-Song-Nop]

@liruifengv 这个文档pr和你的 pr #9 有直接关联。我认为我们可以更多的澄清shift+enter的局限性。