Update Using SSH keys with GitLab CI/CD docs to include File variable type option

Problem to solve

https://docs.gitlab.com/ee/ci/ssh_keys is storing an SSH key as a Variable type variable. SSH keys are files and would be better stored as File type variable.

Further details

Proposal

Update https://docs.gitlab.com/ee/ci/ssh_keys to include (& prefer) File type variable.

Who can address the issue

Other links/references