Add ability to ignore certain classes in the hierarchy

Let's consider the following class hierarchy: ``SpectralAxis(SpectralCoord(SpectralQuantity(Quantity(ndarray))))`` - it would be nice to have a way to document ``SpectralAxis``, including all the methods on ``SpectralCoord`` and ``SpectralQuantity``, but ignoring ``Quantity`` and ``ndarray``.

The issue names no files, tests, or entry points, so first locate the hierarchy-documentation code and trace how inherited members are collected for SpectralAxis. Check how SpectralCoord, SpectralQuantity, Quantity, and ndarray are handled, then add coverage showing that selected ancestors can be ignored while intermediate classes remain documented.

Written by the indexing model from the issue text.