cicd: make per-branch deploy host a gitlab variable

Currently the target host/URL is hardcoded in the YAML. This is kinda ugly and means host changes mean YAML changes.

I propose adding GitLab variables like DOC_SSH_HOST and DOC_SSH_PATH, as DOC_SSH_KEY and DOC_SSH_KNOWNHOSTS already exist.

The job can then be modified to only run when these tokens are present. Which means no failing jobs automagically for forks and others not having the variables set. Yay!