Only run `docs lint` on docs changes

The docs lint job is not long running but a bit of a waste when it runs and there is not change in the documentation or settings.

• Example of pipeline with docs change: https://gitlab.com/gitlab-org/ci-cd/custom-executor-drivers/autoscaler/pipelines/110750839
• Example of pipeline with no docs change: https://gitlab.com/gitlab-org/ci-cd/custom-executor-drivers/autoscaler/pipelines/110750602