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.
Edited by Achilleas Pipinellis