Document Extensions to External Types Using DocC

[theMomax (Max Obermeier)]

Just a quick update for those following this thread: An initial implementation is almost ready to be merged, but I need your help deciding a last specification detail:

Dealing with Nested Types when Documenting Extensions with DocC

A couple of months back I started a pitch on how to Document Extensions to External Types Using DocC. While there will be a few PRs with improvements later on, the initial version is pretty much ready for merge. However, before that, I need your help once more to decide how to deal with extensions to nested types. This decision influences at what URL the pages for extensions to nested types such as SymbolKit's SymbolGraph.Symbol are located and therefore also influences how these pages can be r…