Add 404 page: for URLs that are not found

[sanjayankur31]

This will come in useful when we remove bits from the old website. Perhaps a modified version of the autogenerated search page would be good:

https://docs.neuroml.org/search.html

Here's how to do it for GitHub pages:

https://docs.github.com/en/pages/getting-started-with-github-pages/creating-a-custom-404-page-for-your-github-pages-site