docs: build documentation for threading-related modules

[Luyten-Orion]

Summary

kochdocs.nim in tools/koch/kochdocs.nim , docDefines now has
--threads:on to build docs for relevant threading API.

[zerbina]

Thank you for the PR! I've changed the title to make it a bit clearer what the user-relevant impact is.

[zerbina]

@Luyten-Orion: The compiler bug causing the koch doc failure should now be fixed (sorry that it took so long). Could you merge the devel branch into your PR?

[Luyten-Orion]

Sorry it took me so long, it's been done now

[zerbina]

All the threading-related API now shows up in the index, but the links in index.html are still broken, as both the threads and channel_builtin modules are included from system.nim, meaning that they need to be special-cased by doc generation. They once were, but this changed in #303.

In my opinion, this can be addressed separately, and I think the PR is good to go.

[saem]

/merge

I created an issue for the fix for the links themselves.

[github-actions]

Merge requested by: @saem

Contents after the first section break of the PR description has been removed and preserved below:

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