[clang-tools-extra] [clang-doc][cmake] Copy assets to build directory (PR #95187)

[Paul Kirth via cfe-commits]

ilovepi wrote:

@petrhosek I'm pretty sure there's a better way to spell this, but I think we need something similar to properly test clang-doc w/o an install step.

cc: @PeterChou1

https://github.com/llvm/llvm-project/pull/95187