Add docstrings and list of exports

[gdalle]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

I'm not sure whether the lack of documentation is a bug or a feature. This is a pretty young and experimental package, so arguably we don't want to expose too much of the internals.

On the other hand it already has 18 registered direct dependents and 492 indirect ones. I would assume those dependents use "the code as the documentation", which means everything is API by default. This PR makes the API more explicit and discoverable.