Implement support for ToC to look for pages nested in directories under the current page.

[KelSolaar]

Hello,

As discussed in #1590, it would be helpful to have an option to consider the pages nested in directories under the current page also related. That would allow Docsets like that of Unreal Engine to present class members for example:

The depth at which the related pages are considered might need to be configurable and require a widget, e.g. slider or spinbox to avoid explosion, e.g. picking the index page and listing 0.5 million objects.

Cheers,

Thomas