Make usage of SSH keys in CI/CD more clear
What does this MR do?
Refactor the CI/CD SSH keys doc. Also address comments from https://docs.gitlab.com/ee/ci/ssh_keys/README.html.
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/25716.
Refactor the CI/CD SSH keys doc. Also address comments from https://docs.gitlab.com/ee/ci/ssh_keys/README.html.
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/25716.