Add section headers to navigation element

This issue was originally opened by @donpinkus via facebook/react/issues/10980

The side-nav only displays page titles. It would be nice if it also displayed section headers for the current page:

For example - what are the life cycle events in React? The closest thing in the nav is "Quick Start > State and Lifecycle". After navigating to this page, and reading for a few seconds - it becomes clear this is a tutorial. Is there a list of the lifecycle events? I'm not sure. I scan through the whole page. Doesn't look like it, but maybe I just missed it. Repeating this process of scrolling through entire pages is time consuming, so instead I just google "react 16 lifecycle events" and end up finding it on someone's independent blog post.

先找出 React 文件網站中的側邊導覽實作，以及目前的頁面標題結構。追蹤導覽項目如何產生，接著判斷目前頁面的區段標題應如何顯示；當相關頁面區段呈現在側邊導覽中，且 lifecycle-events 範例更容易瀏覽時，工作就完成了。