skip docs for lib/fusion (docs already run in fusion repo)

[timotheecour]

fixes following bugs:

• [FIX] Update fusion master commit hash #16630 broke docs for future CI's but CI was green in that PR because of a micro-optimization that avoided running ci docs when koch.nim was changed; I've removed this micro-optimization to avoid future regressions
• avoid running docs for lib/fusion since docs already run in fusion repo (via a one-shot approach using --project instead of 1 compilation per file as was done in kochdocs)
• this will also avoid showing fusion modules in https://nim-lang.github.io/Nim/theindex.html (or fusion files eg https://nim-lang.github.io/Nim/pointers.html), which already exist in their own index and sub-directory: https://nim-lang.github.io/fusion/theindex.html and https://nim-lang.github.io/fusion/src/fusion/pointers.html, and also remove risk of module clashes between stdlib and fusion modules

(note: also fixed ci docs but #16644 was merged in meantime; but the other points remain)

future work: don't copy fusion inside lib/fusion

• un-bundle nim and fusion (which was a root cause for this issue). As I've argued in instead of copyDir(fustion, stdlib/lib), update --path fusion#25, copying over fusion in nim lib/fusion/ is a terrible idea.
  • It breaks repo modularity
  • prevents switching between branches (which have a different fusion hash in koch.nim)
  • affects tooling (as this issue showed)
  • will cause nasty issues if user imports external fusion (since each fusion module will end up as 2 distinct module, breaking exportc and other things), etc

[juancarlospaco]

PR Collision:

• https://github.com/nim-lang/Nim/pull/16645/files#diff-548a3839d9ea07f9b217b687ec4810f720118846907ef7cfa9f0c04a6b1a7dffR18
• https://github.com/nim-lang/Nim/pull/16645/files#diff-548a3839d9ea07f9b217b687ec4810f720118846907ef7cfa9f0c04a6b1a7dffR18

[timotheecour]

wrong link, i think you meant https://github.com/nim-lang/Nim/pull/16644/files ;

but this PR is better, it also prevents future such regressions thanks to the change in .github/workflows/ci_docs.yml :)

EDIT: now that #16644 was merged, i updated this PR to also skip docs for fusion entirely, see PR message for rationale + details)

[Araq]

Merging this now, we can do some followup PRs if it's still wrong.