Find a way to make our API docs cover user-facing APIs only

The EF API docs contain our entire public surface, mixing together the (considerable) provider-facing APIs with user-facing ones. This makes it difficult for regular users to find their way (e.g. #4226).

I'm not sure what filtering the doc system provides, but we know it's possible to at least filter out the Internal namespaces. We could start by specifically excluding certain namespaces which are known to be provider-only... At some point we may be able to also use attributes to indicate that something is provider-facing (or user-facing) and filter based on that...

* #4293