ci: dispatch Gitee branch sync after docs pushes

[yanglbme]

Summary

Add trigger-sync-gitee.yml on the docs branch. A push to docs dispatches sync-gitee.yml on main with branch=docs, so Gitee docs stays in sync.

Depends on #5625 landing on main first.

Type

• Other

Checklist

For new or updated solutions:

• README.md / README_EN.md follow solution/template.md
• If a Thinking block is added, it sits after the method heading, is non-empty, and matches the code in the repo (naive idea vs constraints, bottleneck, key observation, why this method). Thinking is optional.
• All languages implement the same algorithm; unrelated Solution.* files and code tabs are unchanged
• Code is formatted as described in AGENTS.md

Test plan

• Merge ci: sync main and docs branches to Gitee #5625 first
• Merge this PR to docs
• Push to docs and confirm sync-gitee runs with branch=docs
• Confirm Gitee docs matches the GitHub SHA

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