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

[via llvm-commits]

zeyi2 wrote:

I plan to split the `Dockerfile` modification to another PR, in order to avoid potential CI failures (e.g. If the Dockerfile modification is not shipped in advance, after changing `image: 'ghcr.io/llvm/ci-ubuntu-24.04-lint-doc8'` before merging, the CI here will fail in `code-lint` job)

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