Allow pinning certain sections in documentation

先查看 README.md 中的 DevDocs 愿景，然后检查 C++ 和 CSS 等文档集目前如何呈现文档章节。明确每个文档集如何存储已固定的章节，以及排序应如何工作；完成标准是用户可以固定一个章节，并在匹配的页面顶部看到它，同时不影响其他文档集。

Feature request

Summary

In documentations a lot of times there are sections that consistently exist across most pages of the docs, for instance in the C++ documentation a lot of times you find that there is an Examples section that shows how to use the function being documented. I would really like to have the example section appear on the top of the page because a lot of times it does a much better job at explaining what the function does and how to use it than the cppreference cryptic definitions.

Examples

I don't have a screenshot of this feature from another source. I think having a pin button next to the sections is a good starting point. When that button is clicked, the section should be added to a list of pinned sections, and whenever the visited page contains that section, it should be displayed at the top of the page. Of course, the pinned sections will be local to each set of docs, so if you pin the Examples section in the C++ docs, it won't be pinned in the CSS docs. This is both simpler to implement and makes sense, since each documentation set has its own structure.

I think this is a solid foundation to build on. From there, you could refine it further, perhaps by adding a settings section in each set of docs where you can view all available sections in those docs and allow the user to drag and drop them into a list to define their preferred order. However, this would 1) be quite challenging to implement and 2) may not be practical in reality.