docs: add Chinese translations for evaluation documentation

[GoofySatoshi]

为评估文档添加中文翻译。当前评估相关文档仅有英文版本，中文用户阅读不便。
本次变更将以下两个文件翻译为中文：
docs/en/open_source/evaluation/overview.md → docs/cn/open_source/evaluation/overview.md
docs/en/open_source/evaluation/openai_memory_locomo_eval_guide.md → docs/cn/open_source/evaluation/openai_memory_locomo_eval_guide.md
翻译原则：
命令示例和脚本路径保持不变
数据集名称（LoCoMo、LongMemEval、PrefEval、PersonaMem）保持不变
使用清晰的中文说明描述环境配置、安装步骤和评估流程
相对链接已验证在新路径下仍可正常工作
Related Issue: Fixes #1692
Type of change
[x] Documentation update
How Has This Been Tested?
[x] Test Script Or Test Steps
在本地打开 docs/cn/open_source/evaluation/overview.md，确认 Markdown 格式正确渲染
在本地打开 docs/cn/open_source/evaluation/openai_memory_locomo_eval_guide.md，确认 Markdown 格式正确渲染
验证 overview.md 中指向 openai_memory_locomo_eval_guide.md 的相对链接 ./openai_memory_locomo_eval_guide.md 在新路径下可正常跳转
确认代码块、命令和脚本路径与英文原文完全一致
Checklist
[x] I have performed a self-review of my own code | 我已自行检查了自己的代码
[ ] I have commented my code in hard-to-understand areas | 我已在难以理解的地方对代码进行了注释（不适用）
[ ] I have added tests that prove my fix is effective or that my feature works | 我已添加测试（不适用，纯文档翻译）
[ ] I have created related documentation issue/PR in MemOS-Docs (if applicable) | 不适用
[x] I have linked the issue to this PR (if applicable) | 我已将 issue 链接到此 PR
[ ] I have mentioned the person who will review this PR | 我已提及将审查此 PR 的人
Reviewer Checklist
[ ] closes #1692
[ ] Made sure Checks passed
[ ] Tests have been provided

[GoofySatoshi]

@CaralHsi
Hi maintainers, this PR is ready for review.

Also, please note that the base branch doesn't allow the standard merge method. Could you please use squash merge or rebase merge when merging? Thank you!

BTW, following up on my previous message — I just checked the repository again and confirmed that there is currently no dev branch in this repo.

Since the contribution guide asks contributors to target dev, could you please create a dev branch to align with the guide? Or should I target another branch (e.g., main) instead?

Thanks!