Exclude docs CI yaml from danger check

What does this MR do?

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30115, we added a danger check for docs-only changes. This would advise the user of the usage of docs- branches to voide running the whole pipeline, as well as it would fail if a branch has the docs prefix/suffix, but the MR touched files other than docs.

For docs-only branches, we should be allowed to change docs.gitlab-ci.yml if that's the only non-doc file in the MR.