[clang-tools-extra] [clang-doc][NFC] refactor out file helpers (PR #134298)

[Paul Kirth via cfe-commits]

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

Since this just moves common code I think this is fine for now. We should revisit some of the interfaces though, especially the ones that return SmallString<128>.  I think there is also a Path API that handles relative paths, so we can perhaps drop that entirely?

Also I know I suggested File.cpp, but maybe FileUtils.cpp is a more apt name? Now that I look at the content again?

https://github.com/llvm/llvm-project/pull/134298