Some members would not show up due to not being mocked. We do not need
the mocking anymore, because we can now properly use intersphinx to
refer to RTC-Tools's documentation.

To avoid _all_ members of e.g. an OptimizationProblem showing up, we
do not rely blindly on automodule, but also use autoclass to
specifically document the relevant members of the Mixins (and nothing
else).