Docs feedback: add_remove_clusters.html self signed certificate

Problem to solve

https://docs.gitlab.com/ce/user/project/clusters/add_remove_clusters.html

When following these instructions to add a already existing cluster there is no mention of how to inject a CA cert into the runner container. There are some docs available at https://docs.gitlab.com/runner/install/kubernetes.html#providing-a-custom-certificate-for-accessing-gitlab but they are for if you deploy the gitlab-runner using the helm charts