sagemathgh-38401: Fix a bug in version selector of documentation

    
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The version selector should work for

1. documentation in online sites
2. documentation in local sage install
3. standalone tree of the documentation (such as github action artifact
doc.zip)

The version selector javascript code works fine with the cases (1) and
(2), but currently it fails with (3).

Tested with doc.zip in
https://github.com/sagemath/sage/actions/runs/10032161715

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### ⌛ Dependencies

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URL: sagemath#38401
Reported by: Kwankyu Lee
Reviewer(s): Matthias Köppe