Add page pointing to API docs

[penelopeysm]

People go to docs and they don't see API docs, so they might well think that it doesn't exist. That's not true though. It does exist, it's just hard to find.

We should have a page that tells them where to find it.

[shravanngoswamii]

Previously, we had Library API in navigation bar, now we are directing Library to that new library page! So maybe we can add a new nav item API Docs or API!

[penelopeysm]

I think this is mostly solved by TuringLang/turinglang.github.io#114