Generate artifacts for manpage for eoscta-docs

We should have a job that generates the .md man pages as an artifact. This way the .externals-script script in https://gitlab.cern.ch/cta/eoscta-docs/-/blob/master/.gitlab-ci.yml?ref_type=heads will be able to use these artifacts using needs:project (https://docs.gitlab.com/ee/ci/yaml/#needsproject).