docs(contributing): document the take and untake issue commands

[jackeyfaker77]

Problem

CONTRIBUTING.md and CONTRIBUTING.zh-CN.md tell contributors to claim an
issue in a comment, but they do not document the repository's exact
self-assignment commands.

The workflow in .github/workflows/take.yml only runs when the complete
comment body is exactly take or untake. A natural-language comment such as
“I'd like to work on this issue” therefore does not assign the contributor and
does not explain why nothing happened.

The self-assignment behavior was introduced by #3774 for #3773, but its
user-facing commands are currently only discoverable by reading the workflow
or the old issue.

Desired outcome

Update both contribution guides to document that:

• a standalone take comment self-assigns an unclaimed issue;
• a standalone untake comment removes the contributor's assignment;
• other claim text does not trigger the workflow.

Keep the English and Chinese guides synchronized. This issue does not propose
changing the workflow behavior.

Alternatives or workarounds

Contributors can discover the commands by reading
.github/workflows/take.yml or the closed implementation issue, but the
contribution guide should explain the supported claim mechanism directly.

[jackeyfaker77]

take

[jackeyfaker77]

I'd like to work on this issue.
Plan:

• update CONTRIBUTING.md with the exact take and untake commands;
• update CONTRIBUTING.zh-CN.md with the matching Chinese guidance;
• keep the change documentation-only and avoid changing workflow behavior;
• run formatting and ASF header checks.

计划：

• 在 CONTRIBUTING.md 中说明准确的 take 和 untake 命令；
• 同步更新 CONTRIBUTING.zh-CN.md 中的中文说明；
• 将改动限制在文档范围内，不修改工作流行为；
• 运行格式检查和 ASF 文件头检查。