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.

Start with src/components/LayoutHeader/Header.js and the content/docs structure, then review how the current active header link is determined. Compare the proposed /api move, redirects such as redirect_from, and separate sidebar structure. Done would mean an API header link that opens the reference section without breaking existing links or active navigation styling.