[clang-tools-extra] [clang-doc] switched from using relative to absolute paths (PR #93281)

[Paul Kirth via cfe-commits]

https://github.com/ilovepi approved this pull request.

LGTM, but can we improve the check for RootPath somehow? I feel like there should be something we can match there. 

https://github.com/llvm/llvm-project/pull/93281