List child classes in the sidebar

[st0012]

With #1183, it's now easier to navigate up an inheritance chain, which is great. But it also makes the lack of downward navigation apparent. Therefore, I think it may be good to also support listing child classes in the sidebar.

• I think we can start with just 1 level of child classes
• For Object, we may need some special handling?
• Unlike the ancestors list, child classes should probably be folded by default