[🐛 BUG ] Search Modal is getting overlapped by Sticky Nav

### Hi team! 👋 David here

Found this error on the docs page 🐛

So usually when you click on the Search bar or use `Ctrl+K` or `Cmd+k` you'll see something like this ⬇️


But, when you scroll just a little bit down to read the documentation and do the same thing, the Sticky Nav which indicates the current docs section where you currently are overlaps the Search Modal like this


### Expected behavior
I would expect to see something similar as the first image like this


在文档页面上滚动，然后通过搜索栏或 Ctrl+K/Cmd+K 打开 Search，以复现该问题。检查 Search Modal 和 Sticky Nav 的入口及其层叠关系；当 modal 完全覆盖 nav，并且在滚动后仍可使用时，即表示完成。

由索引模型根据 Issue 内容生成。