[llvm] [Docs] Migrate LLVM docs to furo (PR #184440)

[Jeff Bailey via llvm-commits]

kaladron wrote:

In the libc docs, I added some JS hooks:

https://github.com/llvm/llvm-project/blob/4bd6175f718f032cc391de9263a3d4c14ae5109d/libc/docs/conf.py#L137

These give a hover over code blocks for easy copying.  There's a sphinx plugin to do this, but I had no idea how I would go about getting one of those added to the builders, so I did this instead.  I'd love to see something like this be consistent across the sites.

https://github.com/llvm/llvm-project/pull/184440