[style] Some page style refinement/re-design is needed.

<img width="176" height="672" alt="Image" src="https://github.com/user-attachments/assets/12a73453-dba4-4146-b474-af623b00ddeb" />

Page here: [swappable](https://cppdoc.cc/cpp/language/named_req/swappable/)

Also, is it possible to make the sidebars foldable? Since I may have no time to do this, I just leave an issue and comments here.