[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:

Could you run a test (can be in a parallel branch) that clang-tidy CI still works

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