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.

まず、React ドキュメントサイトのサイドナビゲーションの実装と、現在のページ見出しの構造を見つけます。ナビゲーション項目がどのように生成されるかを追跡し、次に現在のページでセクション見出しがどのように表示されるべきかを判断します。関連するページセクションがサイドナビゲーションに表示され、lifecycle-events の例をより確認しやすくなれば、作業は完了です。