Consider cleaning up the repository root directory

[moluopro]

The repository root currently contains many small and possibly outdated files, which makes the GitHub homepage quite long before the README appears.

Would it be possible to move related small files into dedicated subdirectories, and archive obsolete files under a legacy/ directory?

[moluopro]

It might even scare off newcomers — just kidding :)

[martin-frbg]

I agree we could move or remove the "historic" GotoBLAS files and remove some legacy CI configurations, but that would only gain something like ten to fifteen lines. Anything beyond that would mean reshuffling code just to cater to the current github layout, and its effectiveness would still depend on screen size. I'm not convinced that it's worth it.