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Description
404 might eventually happen: broken links, versions, documentation refactors...
It would be better to show something nicer, as described in https://help.github.com/en/articles/creating-a-custom-404-page-for-your-github-pages-site
I would go for:
- Show the actual index/TOC on the left
- Show a 404 error on the right
The generation of this page seems could be generated automatically.