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

[Nico Weber via cfe-commits]

nico wrote:

Sorry, I'm confused, how is this build-system dependent? The patch doesn't change any cmake files, and it changes the behavior of an existing test.

I'm not saying it can't be build-system dependent, but it isn't clear to me why it is. Do you have a few more words on what's happening?

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