Explicit APIs for each "level" of documentation

[mortenpi]

DocumentationGenerator can generate multiple versions of the docs for a given package version: fallback readme + API, documenter docs, just the HTML readme.

Right now, that's all opaquely handled by the main build command. We should refactor the API such that it would be possible to explicitly request the generation of certain "type" of docs. This would create better interfaces, and also hopefully assist in testing.

We should also still retain the fallback logic, where we try the most complex option first, and then generate less sophisticated docs if that fails etc.