[clang-tools-extra] [llvm] [Github][CI] Add `doc8` for clang-tidy documentation formatting (PR #168827)

[Baranov Victor via llvm-commits]

https://github.com/vbvictor commented:

I think there is too many churn in current PR because it refactors both clang-tidy linter and adds new doc8.

Can we first make patch refactoring clang-tidy linter into LintHelper and then add doc8 on top of it?

https://github.com/llvm/llvm-project/pull/168827