[llvm] [Github][CI] Introduce `doc8` to `code-lint-helper.py` (PR #172123)

[via llvm-commits]

zeyi2 wrote:

I plan to test the following scenarios:
- `doc8` and `clang-tidy` both succ
- `doc8` and `clang-tidy` both fail
- `doc8` fails, check whether the comments can be updated
- `clang-tidy` fails, check whether the comments can be updated

(But I think it would take some time before I can finish the testing)

https://github.com/llvm/llvm-project/pull/172123