Get source code links working for non-SwiftCrossUI doc targets

[stackotter]

The documentation site now includes documentation for each of the 5 backends, but for whatever reason these targets are missing source code links. I assume it's something to do with the symbol graphs getting generated on a different machine to the one rendering them.

The SwiftCrossUI target is the only one that has working source code links at the moment. None of the other backends display source code links whatsoever.

[stackotter]

There's another similar, possibly related, issue with linking between targets. All of the SwiftCrossUI symbols referenced from the backend target documentation pages don't link back to the SwiftCrossUI target (they aren't links at all). E.g. references to AppBackend don't link to the AppBackend symbol's documentation page.

I tried adding the --enable-experimental-external-link-support flag when compiling SwiftCrossUI's documentation and --dependency SwiftCrossUI.doccarchive when compiling the backends' respective documentation, but that didn't seem to have any effect. If you look at the output of xcrun docc convert --help, you'll see that the descriptions of those command line options do sound like they're meant to achieve this exact goal.