[Feature Request] Nest types declared within classes in TOC

Imagine a class defining another type within:
```csharp
class Operation {
public enum OperationState { Pending, Completed }
}
```
Currently, the navigation sidebar will list both `Operation` and `Operation.OperationState` as siblings. This doesn't really match the true relationship, where `OperationState` is *not* an equal of `Operation`, but has more of a "subordinate" relationship.
A type declared within a class is defined in a way that makes it relevant *within* the context of the class, but not outside of it.
Also, very subjectively, I prefer nested lists over long ones.

I'd propose a config setting, `"nestClassSubTypes"`, for example. Setting this option would nest sub-types (recursively) under the class they are defined in. Clicking the "parent" class entry would still open the normal class page, so no additional TOC entry or page files should be generated.