Explore adding top-level "API" link to header nav

This discussion thread on Twitter provided feedback that the reference section of the docs may not be very discoverable due to the new website's accordion sidebar.

@gaearon proposed we could do something similar to what Jest does and add an "API" link to the header that directly opens the first page of the reference section (eg React Top-Level API).

Unfortunately this is a little more complicated than it would seem at a glance because of:

• How we determine which header link gets the "active" style. If we added a new "API" link, both it and "Docs" would share the same "/docs" URL path. (Jest side-steps this issue by not applying "active" header link styles.)
• We try to maintain symmetry between the location of markdown content (eg content/blog, content/docs) and the top-level nav structure (eg "Blog", "Docs")

Unfortunately we can't rely on anything dynamic (like markdown frontmatter tags) to help with the "active" link styling because the header and footer are part of the "static" Gatsby layout. (Technically we could move them into the templates but this doesn't seem ideal.)

We could move reference docs into a new folder (eg "content/api") give them a unique URL prefix (eg "/api") but we'd need to leave backwards-compatible redirects (eg redirect_from like here) in place so that no bookmarked links were broken. This would also require separating "Docs" and "API" sidebar structure.

从 src/components/LayoutHeader/Header.js 和 content/docs 结构入手，然后检查当前活动的 header 链接是如何确定的。比较拟议的 /api 移动、redirect_from 等重定向，以及独立的侧边栏结构。完成的标准是：API header 链接能够打开参考文档部分，同时不破坏现有链接或活动导航样式。