Document public / internal API split

[SWilson4]

We have two liboqs APIs: the public one, exposed to users via oqs.h, and the internal one, used for non-exposed common code (e.g., SHA3). We should document this API split for the benefit of both users and developers.

Feedback welcome on where to put this documentation. It seems like similar docs live in the wiki at the moment---is this the best place or should we create a tracked markdown file within the repo?

[baentsch]

This must be in the repo, not in the Wiki; how otherwise will different API (concepts) be winding up in release documentation?