[clang-tools-extra] [llvm] [clang-tools-extra][docs] Use Furo theme to match clang (PR #222196)

[Baranov Victor via cfe-commits]

vbvictor wrote:

Ok, I'm stupid: there is a slider on the very bottom to scroll this list, but still I wish we could use all that free blank space I had to make list a little wider.
Maybe .css style could be tuned a little bit?

https://github.com/llvm/llvm-project/pull/222196