Extensions/implementations from another modules in multi-module documentation

[ilya-g]

When combining documentation of multiple modules, Dokka resolves links between them, so for example the types of a core module referenced in the signatures of an extension module get proper links to their respective documentation pages.

However, there's no information in the core module documentation that dependent modules could have some extensions for its API.
I'd expect to see at least the following references to the dependent modules:

• extension functions/properties for the types in the core module. For example, asObservable extensions for Flow from rx2 and rx3 modules are not shown in the Flow index page. Beware that since they come from different modules/packages, they shouldn't be merged in the same overload group in the index page.
• inheritors of types declared in the core modules. For example, the class HandlerDispatcher should be shown in the list of inheritors of its base class MainCoroutineDispatcher

[IgnatBeresnev]

Similar request about inheritors: #1960