Revamp API docs

[takluyver]

This makes the API docs shorter and (hopefully) more useful.

• Don't add inheritance diagrams to all modules. I've manually added them to the docstrings where I think they're useful.
• Don't include undocumented and inherited members in class documentation. Links to base classes are still included.
• Use fewer subheadings, so the overview on this page is shorter. Classes can still be found in the index.
• Add an @undoc decorator to exclude classes and functions from the documentation.

Alternative to #2383. This should require much less manual maintenance than that one would.

[takluyver]

This also reduces the number of errors showing up while building the docs, because we're not trying to document __init__ for classes that don't have it.

[takluyver]

On the grounds that no-one's ever interested in docs, I'll put a soft deadline on this: if no-one objects by Thursday, I'll merge it.

[Carreau]

It's ok for me if you want to merge earlier.