docgen for `threads` and `builtin_channels` links broken

[saem]

Threading and channels related documentation is now generated again, as part of #1464, but due to a docgen bug the documentation for the threads and builtin_channels modules, which are included from the system module do not work.

This used to work with the legacy docgen feature, but that has since been removed, see: #303. Update the existing docgen to get the links working again from the generated index.html.