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

[Baranov Victor via cfe-commits]

vbvictor wrote:

Same happens on "normal" screens (13'' air):
<img width="2916" height="1768" alt="image" src="https://github.com/user-attachments/assets/297e0cf6-87da-41ec-9cc5-252059cfda83" />




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