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.

Comece localizando a implementação da navegação lateral e a estrutura atual dos títulos de página no site de documentação do React. Rastreie como as entradas de navegação são geradas e, em seguida, determine como os cabeçalhos de seção devem aparecer na página atual; o trabalho estará concluído quando as seções relevantes da página estiverem representadas na navegação lateral e o exemplo de lifecycle-events estiver mais fácil de consultar.