add documentation in code and developer docs indicating that APIs are internal #3306
Description
Observed behavior
We're starting to see people in the community build external apps for integrating with Kolibri, either by asking us for documentation or by inspecting the source directly.
Expected behavior
We should dissuade people from building third party apps on the current internal API, and make clear that a stable, public API is on our roadmap.
User-facing consequences
If third-party devs create apps under the impression that our internal APIs are stable, they will be frustrated when those apps break because we updated the APIs.