at: add Simplified Chinese translation

[zhuhongboAAA]

Checklist

• The page(s) are in the correct platform directories: common, linux, osx, windows, sunos, android, etc.
• The page description(s) have links to documentation or a homepage.
• The page(s) follow the content guidelines.
• The page(s) follow the style guide.
• The PR contains at most 5 new pages.
• The PR is authored by me, or has been human-reviewed if it was created with AI or machine translation software.
• The PR title conforms to the recommended templates.
• Version of the command being documented (if known):

Reference issue: #

[tldr-bot]

Hello! I've noticed something unusual when checking this PR at commit
1b44017920ad56b7d01abb9109edbc909263f42c:

• The page pages.zh/linux/at.md is outdated, based on number of commands, compared to the English page.
• The page pages.zh/linux/at.md has a page title that does not match the one in the English page.

Is this intended? If so, just ignore this comment. Otherwise, please double-check the commits.

[tldr-bot]

The build for this PR failed with the following error(s) at commit
1b44017920ad56b7d01abb9109edbc909263f42c:

pages.zh/linux/at.md:1 MD041/first-line-heading/first-line-h1 First line in a file should be a top-level heading [Context: "at 命令手册"]
pages.zh/linux/at.md:
Error: Parse error on line 5:
...性任务。执行结果将通过邮件发送给用户。更多信息请参阅：https://mann...
----------------------^
Expecting 'NEWLINE', 'BACKTICK', got 'TEXT'
pages.zh/linux/at.md:1: TLDR106 Page title should start with a hash ('#')

Please fix the error(s) and push again.