[clang-tools-extra] reapply [clang-doc] Add --asset option to clang-doc (PR #96358)

[Nico Weber via cfe-commits]

nico wrote:

Ah, I see now, the failing test is also very new (70ec8419dd7), and it depends on ade28a77ed177 which the GN build doesn't yet have. But my bot points at this change here for breaking that fairly new end-to-end test. I think it's probably because absence of the css file wasn't an error before, but now it is. It seems fine to me to make that an error, but changing that seems somewhat tangential to what this PR does. Maybe it deserves at least its own "Additionally, make absence of the css file in the asset directory an error because $reason" sentence in the commit message?

https://github.com/llvm/llvm-project/pull/96358