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

[Reid Kleckner via cfe-commits]

rnk wrote:

It seems those were caused by the `{toctree}` elements in those files., so I removed them so we don't have empty drop down menus.

Those pages don't have child pages. The entries under clang-change-namespace are document sections, and those appear in the right hand sidebar when you land on the page.

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